rpm increase when brake applied

[72GMC51]

Has anyone ever had the idle increase when you apply your brake.
I have a 350, with a eldelbrock 1406 600cfm cab a weind dual plane manifold, with a 1" spacer, headers, GM performance HEI timing set at 12 degree, auto cam spec int 214 .443 107 lobe center, exht 224 .465 117 / 112. The problem I am having is at idle if I step on the brake the idle will increase 2-300 rpm. I have check the power brakes over and there are no vacum leaks. Also it has a vibration at 1800 that if you hold it there it will come and go. Also when driving it it seems to miss and runs like it is fighting itself. I have tried almost every thing plugs carb tuning etc. I just cannot find a problem.
Thanks
John

[Longhorn Man]

unplug the vac line to the brake booster, and carefully drive it around the block. Listen to your 1800 RPM rev for the vibration.... etc.
If all your symptoms go away... your booster probably has a leak in it.

[scatesracing]

[Arad68]

How much compression do you have? Ceck the gap on the plugs. My truck had this same vibration for sometime after I rebuilt the engine. I even went back to the machine shop and complained about the balancing job the guy did on the internals. My gap was set too wide causing the spark to arc from under the spark plug boot to the head. The RPM increase is the booster. While it may not be leaking, it may be short of vaccuum. Mine does the same thing if I pump the brakes.

[cdowns]

if you unplug brake booster plug the line to the manifold to test

[corn]

Do you have a lock up converter in your trans?

[72GMC51]

Yes it has a lockup converter

Originally Posted by corn

Do you have a lock up converter in your trans?

[72GMC51]

OK I unplugged the brake booster and the vibration is still there, I put in new plugs gaped at 35 as recommended by engine builder, still there. If I connect a vacuum gauge and slowly pull the pressure release and cause a vauum leak the rpms go down not up but if I pull the plug on the brake booster the RPMs go up? both are ported vacuum above the throttle plate. Could there be a leak in the 1" spacer? I have replaced all the gaskets including the base plate and throttle body gaskets checked the float level and check the vacuum advance. It really feels like the engine is going to self distruct if I keep driving it this way. I am going to try and put a fuel pressure gauge on it and see if that changes anything, and try a new distributor module. also disconnected the flywheel from the touqe converter and no differance.
Thanks
John

[Huck]

You may have two separate issues. If you are running HEI Dist, you may want to try running your advance off full manifold vacuum. I had to do that and set it up at 10 degrees advance with a manual trans. Also HEI's like about .0045 spark plug gap.
Take a peek at the tech tips on tuning on GMCPaul's web site. He has some good advice on tuning using a vacuum gauge to get max tune. If you have a vacuum leak, and you were to pull a small vac line, your idle will increase 2-300 rpm. I know it's easier to say then do but a SMC has a kind of unique sound when it is running at correct timing at idle---your cranking should not be tough to turn and not easy either.
Depending on the cam, you may have some difficult developing sufficient vacuum at idle to get the carb the right draw for the idle passages but thay should and would not affect the smoothness at 1800 rpm.
So my guess is---open the plugs a bit, try manifild vacuum and set timing around 8-10 degree advance unless your cam is really hi-perf (or time it to an experienced ear!!) Huck

[hvychve]

has to be booster, most likely, LOnghorn man explains easy steps to check

[72GMC51]

OK I gapped the plugs at 45 and ran it it seems to run better but it still has the same problems. I checked the power brake booster and it is good. I am going to take the spacer out from under the carb just to see if its the problem. The weired part is if I drive it at a steady speed you can feel the mis but if I jump on it it seems to take off pretty well not as well I think it should but will run right up to 5500 and shift at WOT. I adjusted the timing to 10 degrees and tried to adjust the carb using the vacuum gauge but could get NO change in vacuum no matter where I adjusted the idle screws I could get the RPMs to change but not the vacuum. Also the vacuum was really differant this time as it was no longer steady at 16 but was vibrating very bad at about 13 which it never did before. So I am going to remove the spacer and change the carb base gasket again and see what happens. Thanks for the replys.
John

[72GMC51]

So I believe I have found the issue, I have a cracked head, I am going to be taking them both off and bring them to the engine builder, from initial observation it has a water jacket crack as I now have the slightest leak out of the side of the head between #1 & #3 cylinders as shown in the picture I cleaned it off with a scoth pad but at first it was a very fine line , which we both agree looks like was probable caused by the engine freezing which is a mystery to me as the engine has never been without antifreeze and has been stored in a heated garage since it was built, he said he magnufluxed the heads but some how didn’t catch the crack. The truly weird part is I never noticed it, and didnt lose any coolant until just the last day. The other picture is of the plugs which all had oil on the outside of the threads like it was being forced by the threads, very strange never have seen that before. Now we have to disassemble the engine and hope it didn’t cause any other damage. I would like to up grade to a set of Alum heads so if anyone has a set of 89 up L98 alum heads for sale let me know. I would even conceder a set of cast L98 heads.
Thanks
John

[fastom]

That will do it!

[72GMC51]

Well I have sent the engine back to the builder and he found that all eight (8) of the oil rings never seated and the cylinders were all glazed??????????
So he honed the cylinders replaced the rings, put a new set of RHS heads on it put it on the Dyno 330hp 356tq. I re-installed it in the truck started it up and I have the same vibration at about 1800 - 2200 rpm and when I push on the power brake pedal the rpm goes up. When I drive it still feels like it is fighting itself I have disconnected the toque converter which makes no difference the carb that is on it was used during the dyno session same with the distributor, if I disconnect the power brakes it makes no difference. ???????????????????????????????????????????????????????????????????????????????????????????????????? HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[LockDoc]

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In post # 8 you mention that "both are ported vacuum above the throttle plate" If I remember correctly the booster should be direct manifold vacuum, not ported. Somebody correct me if I'm wrong....

LockDoc

[67ChevyRedneck]

What carb do you have? On an edelbrock the brake booster is to be hooked up to the port at the back of the carb.

I have a similar issue, I have a weird vibration at 1000-1200rpms. Can't figure it out. It showed up a few years after the engine was installed and I made no changes to the motor. I'm going to drive it till it blows and replace it with a GMPP crate engine. Funny thing is, other than at those rpms, the truck runs great. I've replaced: Balancer, plugs, cap, wires, rotor, motor mounts (one was cracked), control modules, all made no difference.

[bigsnookmaster]

You might want to try a new distributor.I had a fresh 406 that I put my used HEI with a super coil in.It idled a little rough,and I thought the truck should run quicker than 8.40 in the eighth.Swapped distributor and next week the truck ran 8.05 in the eighth.I never would have guessed a distributor would have given me that much.The truck was still runnin pretty good with the old distributor though.The idle also smoothed right out with the new dist.

[El Jay]

Originally Posted by LockDoc

-
In post # 8 you mention that "both are ported vacuum above the throttle plate" If I remember correctly the booster should be direct manifold vacuum, not ported. Somebody correct me if I'm wrong....

LockDoc

You're are correct, Sir.
I missed that as I was scanning through the post.

[haysonj]

http://www.hotrodders.com/forum/dist...1-a-59033.html Read this thread about vacuum advance it should be manifold vacuum below the throttle plates. the ported advance was an early attempt on smog control engines. Also the brake booster is full manifold vacuum.

[72GMC51]

I have the brake booster is connected to the fitting on the back of the carb and the dist hooked to the lower manifold vac on the front of the carb. I do have a 1/2" spacer below the carb. The distributor is a new GM performance HEI and was used during the dyno test with no vibration of running problems. The plug wires I have were not used though but are new taylor silicone. The builder could not explain the reason the oil rings didnt seat and the cylinders were glazed which is still a concern I have. The compression got so high in the crank case it actually forced part of the rear seal out. I check the pvc system and it was all working properly evan changed the pvc valve twice.
Thanks
John

[haysonj]

Oil rings have nothing to do with high crankcase pressure. It comes from the compression rings. He may not have had the right rings or he didn't properly gap them. I have even had to file the gap less rings before. I don't know your builder but if he made a mistake he may not want to admit it. As far as the vibration it could be the fan, water pump, power steering pump, Alternator, AC Compressor, or flex plate. There is a different feel between a vibration and a high speed miss. I had a bad boot on a spark plug and could not find it. Put it in a dark garage at night and I could see it.