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Flat Spot in Acceleration

FM Former Member
SCAT 2265 engine with Webers...Timing is dead on...Valves are adjusted properly...Run like stink EXCEPT:

When I have been cruising along at a constant RPM (Any constant RPM between 2000-4000) for a period of time longer than 2-3 minutes and go to double clutch on a downshift, when I tap the accelerator to match the engine/trans rotation before the downshift, I have a "flat spot" in the engine speed...Instead of a smooth increase in RPM, I get what can only be described as a flat sounding "fart."

AND

When I've been running as described above and I downshift and attempt to moderately (As opposed to "balls out") increase my speed, I get the same momentary hesitation on acceleration, then it picks up and goes fine.

BUT

If I downshift and get on the accelerator hard before the downshift and/or accelerate hard after the downshift it runs fine....No hesitation at all.

Problem is of course, I can't drive that way every where I go....My straight pipes are already causing some of the local Barney Fifes to give me the skunk eye.

ALSO

This only occurs if I have been cruising at a constant RPM. If I'm running up and down through the gears, accelerating/decelerating, braking, downshifting, upshifting etc it is smooth as silk.

Any ideas? (No Beano jokes please)

Thanks in advance.

TMc
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FM Former Member
SCAT 2265 engine with Webers...Timing is dead on...Valves are adjusted properly...Run like stink EXCEPT:

When I have been cruising along at a constant RPM (Any constant RPM between 2000-4000) for a period of time longer than 2-3 minutes and go to double clutch on a downshift, when I tap the accelerator to match the engine/trans rotation before the downshift, I have a "flat spot" in the engine speed...Instead of a smooth increase in RPM, I get what can only be described as a flat sounding "fart."

AND

When I've been running as described above and I downshift and attempt to moderately (As opposed to "balls out") increase my speed, I get the same momentary hesitation on acceleration, then it picks up and goes fine.

BUT

If I downshift and get on the accelerator hard before the downshift and/or accelerate hard after the downshift it runs fine....No hesitation at all.

Problem is of course, I can't drive that way every where I go....My straight pipes are already causing some of the local Barney Fifes to give me the skunk eye.

ALSO

This only occurs if I have been cruising at a constant RPM. If I'm running up and down through the gears, accelerating/decelerating, braking, downshifting, upshifting etc it is smooth as silk.

Any ideas? (No Beano jokes please)

Thanks in advance.

TMc
FM Former Member
Try increasing the accelerator pump setting slightly. Measure both sides with a ruler to ensure they're equal. Drive and adjust until it's to your liking. I'll measure mine and post back so you can compare, I have 44idf on a 2165. Before this, check fuel pressure, clean filters, and possibly float level. I read somewhere that 90% of fuel problems are electrical, so that may be something to think about.
FM Former Member
Larry - Thanks...Unfortunately, it's not the centrifical advance weights. That would have almost been too easy. I will make a note in my shop manual to keep those lubed just to prevent having to ever deal with it.

Danny - Thanks...I've got the 44idf's also. I had a '62 356 Super 90 that had Weber's and now that you mention it, I had trouble with a similar symptom on that car and it turned out to be accelerator pump adjustment. Different carbs of course but same symptoms.

It's been so long since I worked on air cooled stuff (Or carburetors for that matter) that I have to have a stimulus to get the neurons storing that knowledge base to fire off.

Full week this week so it will probably be next Saturday before I get to look at it again...

Man it was a beautiful week here this past week. Highs in the low 80's, sun every day, kids are back in school so the tourists are virtually gone...I got a LOT of seat time in on virtually deserted roads over the past five days.

TMc
FM Former Member
Terry, ok, 3/8 inch from tip of accelerator pump rod to face of lever. Your measurement should be a little more, since the displacement is more, and the rod pulls the pump diaphragm. Also, RE the distributor, make sure when running through the rpm range that the timing light indicates smooth and non-jerky advance. I know you ruled it out, but check it anyway, it'll make us feel better.
FM Former Member
just read your post. had a similar trouble with a Weber years ago on a Ford Kent moter....turned out to be the emulsion tubes were reversed! it was a very slight flat spot and I only thought of the tube because I had them out earlier to clean them. Swapped them back and it ran great. If that doesen't work and all other tings are good I
d check lthe size on them and buy some that were on either side of that size and try changing them. Remember, the smaller the air bleeds the richer the mix.

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