Engine stumble at high rpm

If you have access to an A:F meter, you could see if it was ignition or fuel (full lean would be fuel, full rich would be ignition).

Keeping in mind the maxim that 95% of all carburetion problems are ignition, I'd start there. If you've got a "known good" set of everything (coil, distributor), swap them out one piece at a time. Standard ignition parts for these cars are not very high quality.

The remaining 5% would be a restricted fuel filter, a bad fuel pump, or some sort of problem with the main jet circuit.

Happy hunting.

something electrical,, my norton had this missing between 3-4K rpm, tried jets, carb fiddling,,,turns out the electronic  ignition module was going bad,,runs like a top now..

Which electronic Disti module are you using?

Gordon Nichols posted:

Which electronic Disti module are you using?

$5 says, "Pertonix".

Complete junk.

Bobby hasn't mentioned if his ignition system is electronic or not that I can see. 

But Stan, you say Pertronix is junk. Why do you say that, please? My engine has a Pertronix Ignitor 2 system and seems to run just fine but I'm thinking of replacing it just because it it's likely been there now for 50,000 km and looks tired. What system would you recommend for a single ignition system ?  Thanks.

@David Stroud IM Roadster D:

Nope, Bobby never mentioned a Pertronix-- but if he's got one, that's the place I'd start. If you are having good luck with yours, I'd leave well enough alone.

As for me... I've got a long history thinking I'm smarter than people on the internet, so I disregarded everybody's Chicken Little bemoaning of the Pertronix, until I nearly burned my car to the ground due to the spark scatter from one. Mine was all over the place with several different modules, even with a locked out distributor. Turns out, I know a lot less than I think I do.

When the CB Performance "Black Box" was introduced, lots and lots of guys were banging on it over on TheSamba. Eventually, it was revealed that the guys having issues were running Pertonix modules. The Black Box isn't perfect, but it wasn't causing any of the issues reported-- the Pertonix was.

If you frequent other forums, Pertonix is pretty much universally vilified as being unreliable, etc. On a Type 1 in particular, the module itself seems very heat sensitive, and when you really look at what they are using as a pick-up (a tiny little cam on the distributor shaft), it's amazing it works at all. I have no dog in the hunt, but I won't run one any more. I can't make myself go back to points (it's 2019, for crying out loud, not 1969), but I won't use a Pertronix.

As far as what to run, I've had better luck with the Compu-Fire, but they've been a part of the same company as Pertronix for a few years. I've got no experience with them at this point. The guys with CB's MagnaSpark distributors like them a lot. They can't be worse. I've got two Mallory Unilites in a box somewhere, one of which I may lock out and use with my Black Box, but the advance was pretty bad on both.

You did a MegaJolt on your Frankenmotor. I'd do that.

Years and years ago, Pertronix were good. Like back in the 80s I ran one on my Scirocco. Nowadays the spark scatter is horrendous and they seem to fail often.

JUNK!

Ok I did some closer inspection.  A couple of things i noticed.  With the car is idling I pulled each plug wire off at the distributor cap one at a time.  Each time the motor stumble a little except on number 2.  Also when adjusting the carb mixture screw on number 2 i could turn it all the way in with no change. Now for number 3.  When I used the snail type air flow meter on the carbs all pulled the same level except number 3, which didn't pull as much.

I switched out the plug wire on 3 with a spare I had.  It didn't help.  I know it was getting spark as I got shocked when pull the wires from the cap.

Sorry Stan, it is a c.f. magna spark electronic distributor.

Swap plug wires and see if the non-responsive cylinder follows the wire.

Pull the cap and inspect the inside of the dizzy and the inside of the cap.

My guess is you have 2 issues.  Idle jet or circuit clog (non responsive air mixture issue) and something electronic (high end issue).

I'd also throw a set of plugs at it.  They're cheap and it certainly cannot hurt.

As for your low air draw cylinder, that is corrected by air bypass adjustment,  beforee adjusting bypass Id clean the carbs very well and recheck, and then if you still have a low draw on one throat just be sure you absolutely understand how to adjust air bypass before messing with it.  Google will be your friend on this one...

Carey, thanks will give those a try. I already switched out the plug wire and the spark plugs were new last year.  I will clean the jets out.

Bobby D posted:

Carey, thanks will give those a try. I already switched out the plug wire and the spark plugs were new last year.  I will clean the jets out.

An engine that's running fine and then develops a high speed miss usually has an electrical issue. Before electronic ignitions, hotrodded VW aircooled engines were notorious for poor ignition component life. Higher compression, higher coil power and higher rpm's were hard on points, plugs and sometimes even plug wires. You'd run a step colder plugs in your higher revving street engine because stock plugs wouldn't last and would cause pre-ignition (which can melt pistons), but the colder plugs would foul if the engine wasn't revved regularly (think regular traffic, as so many of us were running hotrodded Beetles as transportation), sometimes in as little as 1,000 or 2,000 miles.

Just because you put new plugs in it "last year" doesn't necessarily mean they're still fine. I would look at the carburetors only after ascertaining that all ignition components were operating properly.

As Stan has said (and continues to remind us occasionally)- most (of what we think are) carburetor problems are ignition related.

The electronic ignitions available today have really changed the hobby. Even the Pertronix is such a huge step in the right direction. Too bad it's made so cheaply that reliability is so hit and miss that some guys carry a spare distributor (complete with hold down bracket and already timed) and wrench in the car...

chines1 posted:

My guess is you have 2 issues.  Idle jet or circuit clog (non responsive air mixture issue) and something electronic (high end issue).

 

^ That.

My engine is a 2276, Engle 120 cam 1.25 RR, CB Pancheto heads, about 8.5 compression with sidewinder exhaust.  This setup should pull hard up to 5K RPM.  I am running around 28 to 29 degrees of full advance.

attached is the Magna Spark Instructions.  What springs do you guys think I should be using?

Can some give me a steps to go through to check if ignition is the problem?

Thank You

Regarding the high-speed miss:

Put a timing light on the engine, rev it up to 4000 RPM (or wherever it misses, assuming it doesn’t need to be under load to miss). If the timing stays steady and you can clearly read the mark (30°-ish BTDC), you haven’t ruled out ignition, but you’ve established that your P/U and distributor are OK. There are still the cap, rotor, wires, and plugs to think about, but those are just normal wear maintenance items.

If you can’t read the timing because it’s bouncing around: that’s scatter, and it’s an issue. It would either be the distributor itself or the electronic pick up (or both).

If it’s fuel, it’s a sync issue or a main stack working loose, but it’d have to be really bad to cause what you’re describing. 

thanks Stan,  that give me a starting point.

David:  I ran a MagnaSpark original from 1998 til this spring.  Looks like an 009 case with CB's electronic module and dry-pack coil (looks like a big-ish transformer).  Steady spark, decent advance curve but no springs could be found to change the advance curve.

I installed a MagnaSpark II (MS II) last Spring and ran with the silver spring right out of the box and it seems fine.  Took it to a friend and put it on his Sun Distributor tuning machine and he looked at the advance curve with the silver spring and said "That looks really good to me for a 356....  Go with it!" and that was that.  Billet disti case, very steady spark at all rpms and it comes with an HEI disti cap and 8mm wires with ball-ends, if you get the kit.  The kit has a dry-pack HEI coil (you can opt for a billet mounting bracket for the coil, too).

Gordon

I have that full kit.  It is running with silver springs.

It was funny when we checked it.  I took it to Hot Rod Charlie to put it on his Sun Disti machine and asked if I could watch so he sets it all up and gets it spinning and turns to me and asks, "OK, what's your redline?"

"I'm not sure, but I've seen it pull over 6 grand."

It took a while for Charlie's eyebrows to get off the ceiling, but he said "OK..." and ran it to an engine speed of 7K and looked at it.  "Looks pretty decent, but when you get it installed I want a ride in that thing.    6 grand... Sure..."

I don't think Charlie will fit (he's well over 6 feet) but his brother Jim will - Maybe I can take him!  Here's Jim's '32 coupe that he's had for close to 40 years.  It looks VERY period for a Hot Rod.  Charlie's is flamed like the "California Kid":

Well, I think one problem is a bad mixture screw on # 2.  Looks a little bent and chewed up at the tip.

Yah think?   

The good news is that, besides finding that, it brought the o-ring out with it so you won't have to go dig it out.

"Forward, by all means!"

If that's what the tip looks like, I wonder what the seat looks like?

When I first started tweaking on my carbs, I overtightened that screw (it doesn't take much) and it stuck in the seat. When I removed it to see why I couldn't adjust it, the seat came out with the screw stuck on the tip. Duh.

So I took at look at the timing.  With my timing light, it looks like at 750 RPM the timing as about 11 degrees advanced.  I pulled it back to 10 and the motor started popping when I drove it.

Mine’s running about 16° at idle.

Full advance is very close to 32° above 3K

Timing must be set at rpms over 3000 to be sure you are not over-advanced! 

Timing can only be set at idle on a factory line-built machine. We don't have those.

But, after you get it sorted at high rpm  you can note what the advance is at idle. Every one of these distributors will be slightly different.

It's popping because you don't have enough advance.

To partially rule out the wires and prove good the cap and rotor, put your timing light on each wire as near the plug as you can get it. Check all 4, throughout the rev range, especially around 4 grand where your problem area is.

When you ask for help and people freely give it, then it's not followed, it can be extremely frustrating. Especially when the advice giver has been there/done that and often.

Advice: Check your timing at 3-4k rpm. Bobby D: "I checked it at idle." Seriously?

You've no idea how many times I ask guys what their fuel pressure is and the reply is ALWAYS the same: "the regulator is set to 3 pounds". Exactly how do they KNOW? They don't. Go buy/borrow a vacuum/pressure gauge and KNOW.

Danny

My engine was running at 28 to 29 degrees advance before I putted it back to check the idle advance.  I know total advance is the  main thing.  I was just surprised to see my idle advance over 10.  

At 4K under no load I didn't notice and stumble.  You are right about my fuel pressure.  I don't know what it is.  I will check that as well.

For the other problem I was chasing, I removed both carburetors and clean them thoroughly with carb cleaner then compressed air.  I just got them reinstalled and have called it night for now.  I will pick it back up tomorrow.  

Danny

I will do everything you outlined and get back to you tomorrow.

Thank you for the help.

I don't know why I always get sucked into these "troubleshooting from afar" threads, but I do. I'd like to say I'll stop, but I've told myself I would many, many times and I haven't. 

What you are asking us to do, Bobby, is nearly impossible. There's no audio, no video, nothing but a description of symptoms:

"engine stumbling. Under spirited  acceleration the motor seems to drop of subbenly around 4k."

You've got to start at the beginning. Every story has a beginning, a middle, and an end. Did the engine ever run right? Did it suddenly change, or has this been a gradual thing? What are you running for carbs, ignition, etc.?

In my last post, I asked if it did this any time the engine went over 4000 rpm, or if it only did it with a load-- that matters, and you never answered. If it is not load dependent, that's spark almost for sure. If it IS load dependent, then it points to fuel. Does it only do this when it's hot, or all the time? If it's only when it's hot-- to me that would point to an electronic pick-up or a bad coil.

Information is the key here. More information is the only way this (online troubleshooting) has any hope of working.

Every problem with an ICE is one of three things: fuel, spark, or a mechanical issue.

Since 95% of the time the problem is spark, I start there. The timing light thing is a good first step. Once you're confident that the spark is consistent, and happening at the right time, I'd move on to fuel. Are the carburetors secure on the manifolds? Are the manifolds secure on the heads? Are you sure the manifold gaskets are still in one piece, or might they have a leak? Vacuum leaks are not easy to find, and almost always require taking the manifold off to find.

Are you sure you are being supplied enough fuel to keep the carbs full under load? What's the fuel pressure (under load)? Have you taken the fuel filter off and checked if it's plugged? Is your tank a rusty mess? There are inlet screens into the carbs-- have you taken them apart to see if there's crud blocking them?

Only after you are sure of those things would I move to the carbs themselves. Are they synced? Farting around with things at idle helps with everything under the 4000 RPM you say the problem is at. 4000 RPM is a main circuit problem, if it's carburetion. The only time I've ever had a main circuit problem was once when my main-stack loosened up and fell out.

The last thing is mechanical. You'll need to take your valve covers off and carefully note what is happening at the valves. Are some of them tight? Are all of them lifting the same amount? Do you find any broken pieces of valve-spring laying about? Pull the plugs and look at them. Black? White? Wet? Burned off? Then do a compression check on every cylinder.

Guessing isn't troubleshooting, and we're just guessing out here on the interweb. Let us know what you find.

Stan

Thanks for the reply.  I am going to resume work on the car this afternoon.  I have made list of everything recommended to check.  I will report back this evening.

Thanks again to everyone for all the suggestions.

Bobby:  As part of your investigative study, could you also tell us what the number is on the plug?  Like W7/WR7** or W8/WR8**, etc.  That might help, too.

If you have to pull a plug to read the number, could you take a photo of the tip (the working end).

Thanks.

I'm just sittin here trying to figure out which end of a Dell secondary venturri is "up".

(I think I've got that, now.......)

Advance is set to 30 total.  Still some popping.  The car does not seem stumble at 4k with no load.  I put the timing light clip on each wire one at a time and they all look to be firing.  However, when I unplug each wire one at a time the engine will stumble accept for #3 that has no affect at all when unplug at the distributor.  I have a spare Magna Spark and put that cap on the one in the car and there was no difference.Also when I try to adjust #3 mixture there is no affect either way.   I am going to pull the spark plug out of #3 when the motor cools down.  I have been using NGK DP8EA-9. 

I am going to run out and pickup some new plugs.   I am using the right type of plug?

I just spoke to CB Performance tech.  He recommended the NGK D6EA based on my setup.

Well, going from a hotter to colder plug is usually recommended for higher - rev-ing engines so no surprise there.  If you put in the new plugs and it clears everything up it probably means that the DP8 plug that was in #3 was failing.  It happens.  You should also plan on swapping DP6 plugs every 12,000 miles or so, just because they don’t burn off deposits as easily as a hotter plug, but plugs are cheap, no?

If, OTOH, the condition remains after going to the new plugs (and with all the other stuff you’ve done) I would suspect an intake manifold leak on the #3 runner, either at the top or bottom (most likely) gasket.  That is because your mixture screw is unresponsive so that cylinder is super-super lean and not doing much of anything.

Thanks Gordon

I will be able to pickup the new plugs tomorrow morning.  If there is a an intake leak it would have to be where the manifold and head meet.  I checked out all the gaskets last night when I clean the carbs.  I could not get to the manifold bolts on the head.  I think the thins cover them up.  Hopefully the new plugs will resolve the issue. I will keep you guys posted.  Not much more I can do today without the plugs.

“If there is a an intake leak it would have to be where the manifold and head meet.”

I thought that, too.  The nuts are pretty hard to get at, but are probably 11mm or 12mm.

Try the new plugs first, though.  One change at a time to keep things simple.

Just a small matter of clarification- NGK plugs run opposite most other brands in that smaller numbers are hotter than larger. If 7s are the baseline, 6s are hotter and 8s are colder.

CB recommended a hotter plug, because of his low(ish) compression.

Back to square one. #1valve adjustment, ice cold. #2  compression check: remove all plugs, block throttle wide open, check each cylinder. If they are all within 10% of each other, then move to intake leak. Spray carb cleaner or ether around the intake base while the engine is running. Any speed increase and Bob's your uncle.

No speed increase? Move on to the carbs, but only after verifying the spark is 100% good. And verify fuel pressure.

Thanks, Stan.   I forgot that NGK's run the other way (someone mentioned that on here a few months back - I should-a remembered).  Been running Bosch and Champions all my life and fell into a groove.

Gordon Nichols posted:

Thanks, Stan.   I forgot that NGK's run the other way (someone mentioned that on here a few months back - I should-a remembered).  Been running Bosch and Champions all my life and fell into a groove.

Denso as well. I think it's a Japanese thing.

Ok, here is where I am at:  I feel the ignition system is fine.  I put an old spark plug from last year into #3.  Then I ran the motor pulling each plug wire again one at time from the distributor.  All but #3 resulted in a stumble.  I then, put the plug I had pulled out of #3 and inserted it in the plug wire outside of the cylinder and ran the motor.  The plug was sparking great.   I put all new plugs in each cylinder.  No change.  Are my assumptions about the ignition system correct?

I did a compression test on each cylinder and all came back at 100 except #3 which was 110.

As I mentioned before the mixture screw for #3 had not effect on the engine.  I cleaned the carb again with no change.   I don't think I have any leaks from the carb down to top of the intake runner.  I can't get to the Intake cylinder nuts as they are covered under the tins.

I am still not sure gas is getting into #3.  I think I need to crank it over a few times with the distributor unplugged and then pull the spark plug and see if it wet.

Its an intake leak!!  I sprayed carb cleaner down near where the intake meets the heads and the RPM jumped.   So that has to be it.  I am not sure I can get to the bolts without dropping the motor.