Need Help With Popping Engine

I'm out of ideas! The problem is that I get either misfires or backfires out of the carbs at just over 3000 rpms, sounding like popcorn popping. I don't know specifically what's inside the engine, but was told by the previous owner that it is an 1835. It has dual Kadron (Solex/Brosel H150-2) 28/40 carbs with 130 main jets and 55 idles. I have used spacers under the stock fuel pump to produce 1.5 to 2 psi fuel pressure. It has a Bosch 009 with electronic breakerless ignition with a finned aluminum box mounted on the fender well identified with "Universal Corp" made in USA. Compression is 116/120/122/123 cold and 127/130/127/128 hot. I have set the valves at .006. I have set timing at 28 degrees BTDC at 3000 rpms. I have had the carbs apart and cleaned them with carb cleaner and blown out all passages with compressed air (3 times). The accel tubes seem to have good squirt and I have adjusted jets to max rpm by ear. The car starts fine and runs decent at rest, but try to drive away and just past 3000 rpm's every time you reach for a gear it starts missing (?). Sorry for the long story, but hopefully someone has some more ideas. Thanks!!

clogged idle jet

Put your hand on the top of the carb and see if it moves.... this was happening to me and the left carb was loose on the manifold.....

Exhaust leak? Can you go around the exhaust and see if the bolts are tight?

Open up the mixture screws 1 full turn and see what happens, then:

What color are the plugs? That'll tell us a lot. Pull'em out and compare them to the color chart in a Chilton's, Motor's or Bentley's service manual (check your local library for one). Sometimes they have free Champion color charts at Autozone, et al. Once you do a chart comparison you'll know if it's running rich or lean but beware that, since it's running best on the idle jets and not the mains, you'll spend most of your time at lower RPM's so the plug color may be misleading.

It doesn't sound like electrical, although I would never eliminate plug wires, disti cap or rotor. When were those changed last? If the wires are old they could be arcing to the tin.

Do you have or can you easily get any other main jet sizes? Sounds like the idles are OK, but the mains are off (especially if you've done all that stuff and nothing has changed in the MO). Something like sets from 135 - 150 in increments of 5, or even just 140 and 150 to shotgun it. Maybe even some 155's.

Given the problem with matching plug color up above, could you find a garage or friend (local car club member?) who has a wideband air/fuel mixture gauge to see where those carbs are currently set? THAT would save a lot of trial and error...

Good luck....gn

Thanks for the quick replies! The exhaust manifolds are tight as are the carbs. I do need to spray some carb cleaner to those areas while running though, just to check for leaks! The plugs are stock Bosch, and are relatively new (less than one hour and were gapped to .024/.028 factory spec if I recall. Should the gap be larger given electronic ignition? When I removed the last Bosch set, they were very black and sooty. Rich, correct? So what do I need to do for jet changes? I read on the Kadron/AJ Simms site that Kadrons can't be checked for air/fuel mixture with a gauge due to venturi design. Is this correct? The wires, rotor, and cap look fine, but maybe I should replace them as I don't know their age. I was trying to talk my wife into hanging back over the engine area while I drove to look for arcing spark and to see if one bank or the other was missing, but she just gave me one of those looks. I don't get it???

Ron, Your problem is your fuel mixture is lean. You are not popping at idle because it's correct for that rpm/load. If you have eliminated
air intake leaks then bump up your mains till it goes away.
P.S.--if you do get your wife to peer down the carbs while under load-I hope you like the eyebrowless look because you may get your wish.
P.S.S. if you want to prove the fuel ratio issue--slightly rich'n the ratio by partially covering the inlet and take it for a ride.
Good luck!

Very black and sooty plugs = rich, yes.

Check the size of you main jets and decrease a size or two.

Sounds like fuel flow starvation. Enough gas at idle, but...

ASSUME NOTHING.
Always suspect the supply B4 the carbs. New filter is the first and easiest link.
If its clogged or dirty (backflush into clean clear container), suspect dirty gas. Dont fget the fuel filter screenin the tank !

If that didnt solve it, then
Hookup a temp gravity feed fuel supply,
like a 2L coke bottle 2' above the carbs. Tape it to the rollbar or have a friend hold it while you wind it at least 2/3 throttle thru the gears.
Did the prob go away?
If yes,
Then measure the fuel pump output [volume x time],
w/wo the fuel filter.

Ron, I reread your original post---have you determined if "popping"
is back through the intake? If so then I still think you're lean while running, "popping" through exhaust=rich.

The sooty plugs make sense, if the 55 idle jets are (I suspect) too big and thereby causing the plugs to go black, while the 135 main jets seem too small and are causing it to go lean around 3K rpm when it starts drawing through them.

If it's popping through the carbs (from the original post) then I concur with the posts immediately above - either fuel starvation or generally running lean. Fuel starvation is often pretty dramatic: intake popping, coughing and generally awful running until you take your foot off the gas and then it miraculously clears up (until you stomp on it again!)

So I guess you can change the fuel filter(s) (look for more than one), check for kinks in the fuel lines and then start playing with jets:

Drop the idle jets DOWN one size to 50's (that should make the plugs a little lighter), and then increase the Main jets UP to 140's to start, although it may need 150's or better (yeah, come to think of it - go with the 150's). An 1835cc isn't a huge engine, but it's moving enough air to like larger main jets than what's in there.

gn

Gerd - Oh sure Mr. Mechanic who doesn't know which end of an end wrench to use and doesn't want to.

Tonight I checked out the fuel supply. Tank and screen clear. Line clear. New filter installed. I checked fuel pressure again with a calibrated professional gauge and it was a steady 2 psi. I remember when shimming the pump that any additional shims did not decrease the pressure any further. The pressure was the same from idle to 3500 rpm (started missing/popping thru the carbs at 3100 rpm). I richened the mixure screws one turn and thought it ran a slight bit better, but at this setting there was black smoke coming from the pipes at idle. I checked carb accel pump rod adjustments and found them to be as suggestewd on AJ Simms Kadron site. Tomorrow night I want to spray the carb bases and intake manifolds to check for leaks. What do you recommend for a solvent? I was going to use carb cleaner, but I don't want to mess up the paint on the engine compartment. If I don't find any leaks I will get the recommended jets ordered as suggested above. Is Lowbudget.com the best place to get the jets? Thanks to all!

Everything you've said so far points at mis-matched jets: Too rich at idle and too lean at speed. See my post up above and start by getting the idle jets leaned out. Drop one or two sizes and see what happens, THEN change the main jets (after you have it idling without being rich).

gn

Soon as you get your vehicle problems sorted out, you need to start work on getting a new wife. One who understands that to be a speedster owner may occasionally require some mental and physical gymnastics. I bet you could get lots of advice on that issue from this revered forum.

Gordon: Thanks for the suggestions! I will get jets on the way and report back.

David: My wife is actually a pretty good sport when it comes to my hobby. She (used) to pump the brake pedal for me when I bled brakes (on many different cars). Then a buddy tells her about a Mighty Vac vacuum bleeding tool so she doesn't have to help any more. Then there was the time she was holding a trouble light in the driveway (before we had a garage) at 10:00 pm while I made fiber glass repairs to a Corvette.

Ron.....She's a KEEPER...

Mighty vac? That's what they call that pump nowadays?

Shouldn't his fuel pressure be 3.5psi?

I'd also start with the easy stuff, plugged idle jets, or carbs that are out of sync at rpm.

Sure, he could take the spacer out from the bottom of the fuel pump to bring the pressure back up to 3.5 lbs.

The rest of the symptoms that he's reported point away from clogged idle jets or out-of-sync carbs.

The reason I shimmed the pump was to reduce pump arm travel, thus fuel pressure from my stock 3.5 psi pump. The Kadron website (lowbugget.com) recommends 2 psi max due to the design of the needle and seat. I spoke with a tech at the site today to order jets. He says they typically use larger idle and main jets (than my 55/130 jets) on a 1835 engine. He is going to provide AJ Simms with my details who will then ship me the recommended size jets. Interesting to note, when I measured the jet orifices on the optical comparator we have at work, the idles measured .556mm and the mains measured 1.308mm. Pretty close to what was marked. One more update; tonight I sprayed carb cleaner on the intake manifold bases and tops with no signs of rpm change while running. No vacuum leaks!

Ron:

Your reported symptoms didn't say vacuum leak, especially with the "normal" idle.

I still say 45 idles and 150-155 mains. I wonder what AJ comes up with??

fuel pump pressure just fills the bowls with fuel-not very important.
As long as it's positive and not excessive. The reason they (AJ and others) want to limit the psi is because the notoriously crappy fuel
float needle seal that causes flooding and other troubles.

I just received an Email from AJ Simms at Low Bugget and he is sending 58 idles to replace my 55's and 140 mains to replace my 130's. I guess I don't understand increasing the idles when it runs rich at idle, but maybe I can lean it out with the screws and have additional upper flow before the mains come in? I will let you know how it runs in a few days.

hmmmmmmmmmmmmmmm..............

Well, what the heck to I know??

This presents the opportunity for a great learning experience.

For all of us....

Let's see what happens with the new jets.

(sure wish you had an air-fuel mixture gauge in there......)

gn

just a thought---he may think that a little richer idle will help in the transition from idle to midrange. Did he have all the info--plug color/condition?

I received the jets and carb rebuild kits today. I replaced the 130 mains with the 140's and the 55 idles with the 58's. I then readjusted the mixture screws for peak rpm at idle. It now accelerates great thru the gears getting up to speed with no hesitations. I'm shifting at 3000 to 3500 rpms. I'm getting a little black smoke at idle when momentarily revved, but no worse than before the jet change. The remaining problem is that at constant cruise speed (approx 2500 to 2800 rpm) I get a random pop or backfire thru the carbs. Maybe at 5 seconds; maybe at 10 seconds, but not consistent. I didn't have time today to spend much time on it, but will pull the plugs to see what they look like tomorrow. I only had time to drive it about 5 miles. Is this long enough to get a plug reading? I was waiting to put the new kits in the carbs till after I had the bugs worked out, but is it worth going ahead just to know that it has new accel pumps and float seats?

1. I'm getting a little black smoke at idle when momentarily revved
(This is normal as you blip the throttle because the accel pump squirts in a little extra to temporarily richen the mixture),
2. no worse than before the jet change.
(this is because of the above, if you have blake smoke at idle then you have a problem/over rich mixture etc...)
3. The remaining problem is that at constant cruise speed (approx 2500 to 2800 rpm) I get a random pop or backfire thru the carbs.
(when you get a backfire "THRU THE CARBS" not to be confused with exhaust burbling then one bank of you engine is still lean)
4. Maybe at 5 seconds; maybe at 10 seconds, but not consistent.
(I don't understand what you are timing?)
5. I was waiting to put the new kits in the carbs till after I had the bugs worked out, but is it worth going ahead just to know that it has new accel pumps and float seats?
(the accel pump and float seats aren't causing your problems. In fact the accel pump isn't even working while you problem is occurring. )

Keep looking---Good Luck!

4.) Explanation........... The backfire thru the carbs happen at inconsistant times. It may run smooth for 5 seconds and then backfire. It may then run smooth for 10 seconds and then backfire. The next time maybe 8 seconds apart. The backfires happen anywhere from 5 seconds apart to maybe 10 seconds apart. I pulled the plugs tonight and they all had a smooth black velvet coating. Nice and rich. Note though that I only drove 5 miles or so after changing jets today.

Ron......Just reviewing this post, when it struck me that a very small amount of water in one of the float bowls could cause this .....
Lane Anderson chased carb problems for weeks until he rebuilt his carbs, and ran several bottles of "gas dry" through the system....I occasionaly develope Lane's problem if I drive in the rain for extended periods of time....The gas dry works for me without the rebuilds....

Just a thought......

This morning I pulled the electronic ignition hall effect device and cleaned underneath to make sure the contact was good. I also checked the mechanical advance arms to make sure they weren't sticking or springs broke. I rechecked timing afterwards which was the same. I then double checked the idle adjustments and played with the mixture screws for awhile. It is now back to it's original pre-jetting change problems under acceleration. It even wants to backfire intermittently thru the carbs when revving off idle (didn't do that yesterday). I don't believe what I did this morning should have caused a change. I think I'll pull the idle jets and clean them in an ultrasonic sink at work tomorrow, however this never helped before with the old smaller jets. Maybe fuel quality?

Can you convert that distributor back to points? Or at least get a new electronic module for it? I would not trust the module you have.

Bruce' advise just above is right on, and I agree that your jets aren't clogged, nor is the problem in your accel. pumps (although the pumps may be set a tad rich).

Still sounds lean and, at 2800 rpm you'll be just transitioning from your idle jets to the mains. You're already pretty rich on the idles, but the decision will come down to inceasing the size of the idles or mains by one or two sizes. I would go up one size on the mains and try that (but they're cheap - get the next size after that, too, for both idles and mains).

gn

It's beginning to to sound like either water or crud in the carbs, as Leon said. My recent tear-down included cleaning out the tiny little progression (progressive?) holes in the venturies. There are 5 or 6 of them in the throat, arranged in a matrix. We used a small piece of wire to poke into them. After major cleaning, doing the little holes, and opening the idle bleed screws up to one turn each, the car runs as well as it did when I first got it - before the goop from the gas tank clogged everything up. Not sure what step worked the magic, but I'm happy.

I don't think it's junk in the bowls as I've had the carbs off 2 or 3 times using carb cleaner and compressed air and I blew each and every port. Never saw any contaminant. The car has always been stored in an air conditioned (hot/cold) garage and I've never washed it or drove in the rain, so I don't believe moisture is the problem. Looking on the gas tank, I can see about 1/3 or more of the inside and except for a few spots of minor rust stains, (really discoloration and not surface crust) it looks pretty clean. I have a can of urethane tank sealer sitting on the shelf and have given thought to coating the inside. The coating is supposed to be compatible with everything except MEK. This evening I changed the 58 idle jets back to the original 55's, but after readjusting the mixture screws, it didn't seem to make any difference in the way it ran. Can someone explain to me how changing idle jet sizes changes the way the engine runs. When I go to larger jets, I run the screws in. When I go to smaller jets, I turn them out. Doesn't the fuel/air ratio end up being the same? Gordon, I like your logic on larger mains. I'm thinking about taking my old 130's and drilling them on the lathe to .059 which would be a #150. Thanks to all for staying with me on this!!!!

You may be right about the contamination, Ron, but I cleaned my carbs out repeatedly, with mixed results. This last time I discovered and cleaned those little progressive holes, and it's like a new car. Have you noticed an pealing off of the black coating on your floats? I had to replace mine because of that. Not sure if your carbs have the same sort of floats as mine, but it was certainly a problem.

Harried Ron wrote: "Can someone explain to me how changing idle jet sizes changes the way the engine runs. When I go to larger jets, I run the screws in. When I go to smaller jets, I turn them out. Doesn't the fuel/air ratio end up being the same?"

Ahhh......a common misconception, easily explained.

The idle jets set the ratio of fuel to air in the system. The jet is the ONLY device that sets the fuel to air ratio.

The mixture screw is simply a volume control for the idle circuit at idle. Since the throttle butterfly is (essentially) closed at idle, the low pressure (vacuum) area is below the butterfly. Those progression ports Lane is talking about are all above the throttle plate. When the plate is closed, the air mixture screw sets the air/fuel idle volume (already mixed by the idle jet) and dumps it into the carb throat just below the butterfly.

Think of that air mixture screw as the first (and lowest) of the progression ports (progressing from idle to where the main jet begins to flow.

As the throttle is opened, it moves the vacuum area higher up in the carb throat (following the upper, leading edge of the throttle plate) and so begins to draw through the progression ports (usually they're in a vertical line) pulling more and more fuel/air into the throat until you open the throttle enough to reach the main jets.

That's it...

gn

GOOD NEWS! It runs great now. I'm not sure if it was one thing or a combination of things. I took a day off from work yesterday and pulled the gas tank, dumped the gas, rinsed the tank, blew out the fuel line, an cleaned the carbs for the 4th time. I reduced the accel pump rod travel by 3/16" as per AJ Simms web site directions and used a small aluminum wire to check all the carb orifices. I also increased the plug gap from .026 to .046 (I'm going to start a new topic post for plug gap). The plugs are cleaning up nicely and it was a blast to drive today (I took off the second day just to enjoy as it is 78F here in So. Illinois). When I had the carbs over a bucket, I noticed that when the accel pump rod is held in mid stroke, gas continues to siphon from the bowl to the tube outlet into the carb throat. Is this normal? It could explain why I was running so rich. Thanks again for all your suggestions! This is a wonderful sight.

Sometimes it doesn't matter WHAT you did when you shotgunned the situation, just that the problem has gone away and you can now enjoy the car.

THAT's what's important, 'cause everyone's car is different.

Glad you're straightened out!!

gn

Oh, I can't help with the fuel flowing from the accel pump port - don't know if that's right or not - anyone??

Fuel continues to flow---hmmmmmm. I'm not familiar with your particular carb but....You used the term "carbs (plural) over the bucket implying you saw it with both--so I would "guess" you don't have the same problem on two differnt carbs. But the way accel pumps usually work (as I understand) is;
1. when at rest (diaphram retracted) the fuel chamber gravity fills with fuel(just like a brake master cylinder)
2.when actuated the diaphram squishes (raises the chamber pressure)the fuel through the port and into the carb throat. But (also like the brake master cyl.) the supply fuel port to the accel chamber has to be blocked during operation or you would be sending fuel back into the fuel bowl also. They could manage that by using different sized passages but don't know.

and another thing..........................I had some free time so I researched the above rather than "shoot from the hip"(I'm best at that though). Take a look at this link ------as above the fuel direction is managed by check valves and should not be able to continually siphon the bowl through the accel tube.

http://www.lowbugget.com/main_page.html