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'86 Brat (non-turbo) sudden rpm surge at start


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#1 apparition13

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Posted 20 November 2006 - 11:49 AM

Before: To start, when colder ouside: Give it some gas, turn the ignition. Engine would rev to 2k+, then settle arount 1.5 or so while it warmed up. If I started driving too soon, it would hesitate (at lower rpms) until it had warmed up. If I let it go until the rpm dropped to a normal idle, it drove just fine. I mentioned this to my mechanic (I just drive), and he adjusted the choke

Since: Same starting procedure, but the engine will only rev to 1.5 or so, and then die unless I press the accelerator to hold the revs above 1.2 or so. After 20-30 seconds I can let off the accelerator and the car will hold idle on it's own. If I pull away before it warms up to the point it drops to normal idle, it drives fine, no hesitation.

Last night: Same starting procedure. Gave it some gas, turned the key, and...

EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE...

Immediately jumped to 5500 for a couple of seconds, and just as quickly to 0 and died. I sat there for a few seconds wondering if I had just killed the engine, then tried again. This time it went to 3500 or so, then came down to a more normal warming up rev, except, about more times before it warmed up and dropped to a normal idle, it jumped to 5500 again for a second or two (I may have felt/heard a click right before, but I'm not sure), and then dropped to where it was before the surge (but didn't drop to zero and die). I drove it a couple miles to a friends house, spent 2.5 hrs there, then started it up again to drive home. Started normal, drove fine, until a mile or two later when it suddenly surged to 4000 for a second or two while in gear (the car didn't jump forward though), and that was it until I got home (which was a mile or so later).

My mechanic has no idea what the issue is, but I'm going to drop it off in a bit here. He'll keep it overnight and look at it in the morning when it's cold, but I thought in the meantime I'd try posting here to see if anyone has any idea what may be the problem.

#2 apparition13

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Posted 20 November 2006 - 12:51 PM

Today: Started the car, had to press the accelerator to keep the revs high enough to keep the engine running, but after about 15 seconds it suddenly jumped to 5300 (no click, must have been my imagination last night) or so and stayed there. I had to turn it off to make it stop. Started it again, and it was idling fine at about 1.2 -1.5k, a couple of minutes later it jumped to 5300 or so again, but unlike last night it didn't do it for only a couple of seconds and then drop back to where it was, I again had to shut it off. Started again, it finished warming, and I drove it to my mechanics without further problems.

#3 Frank B

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Posted 20 November 2006 - 03:53 PM

It has to be the carb, choke kickdown linkage, throttle cable, or the throttle return spring. Start with spraying carb cleaner on all the linkage, make sure the linkage operates smoothly, check that the return spring and throttle cable aren't getting hung up on anything.
The throttle cable may be frayed at the end and getting stuck. The throttle plates may be getting hung up too due to worn throttle shafts. or there is some wierd vacuum leak.
it may help to halve someone operate the thottle while your looking at the linkage and such.




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