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Wed May 21, 2014 8:36 pm

So Ive been messing around with my engine (2110 w/ dual 40 idf's)and still cant get it to run right. The motor has 2000 miles on it. It has been running great up until a few weeks ago and it started to back fire a bit after letting off gas. Then it got really bad to where the idle at a stop would go up and down (Like from 820 - 870 and back down).
I did find that my manifolds were loose at the heads and replaced the gaskets, tightened all the screws.
Ive gone through the valve adjustment twice to make sure and then adjusted the dual 40 IDF carbs to 5mg. Then the timing to 8tdc @ idle. I have a magna spark ignition.

When I rev up the engine and let the pms fall, they go below 800 and back up to 830 and then go up and down.

Cant seem to figure whats the matter. Could it be the spark plugs? It has a new cap and rotor and wires.

Any help would be greatly appreciated.

Wed May 21, 2014 8:46 pm

Confirm your fuel pressure. What did you set the float height to?

Does it do this while braking only?

Wed May 21, 2014 8:49 pm

fuel pressure is about 2-3psi and i didnt adjust the float. didnt think i needed to do this. Any instuctions on how? Thanks

Does it while at a stand still. Also when I rev it up the RPMs fall and then climb back up to normal.

The car will idle normal for about 10 seconds and then jump, but it is very suddle. After I drive it around for a bit the rpms fall to about 7000-7500 rpm at idle.

Wed May 21, 2014 10:57 pm

jz56oval wrote: . After I drive it around for a bit the rpms fall to about 7000-7500 rpm at idle.

That is a very! high idle... :P

Try looking for another vacuum leak, do you have a vacuum advanced dizzy?

Thu May 22, 2014 2:13 am

does it stabilise if you detach the linkages from the carbs? Either the A/F mix is unstable, or spark or linkage, probably a combo of all 3... proceed by elimination.

Thu May 22, 2014 2:53 am

The more i think of it, the more i'm thinking vacuum leak. Are you using the stock metal gaskets for the manifolds? Don't, and check for leaks, because you can't just re-tighten them, you need to change them each time. Get paper-fiber ones instead.

Thu May 22, 2014 6:10 am

I'm running a electronic ignition from cb performance. Sorry I didn't mention that I replaced all gaskets with paper gaskets. The problem happens even when the linkage is not attached to the carbs.

The carbs are really responsive if u get on it and don't big down. Just after a Rev up is the problem and at idle.

Thanks

Thu May 22, 2014 6:14 am

FreeBug wrote: The more i think of it, the more i'm thinking vacuum leak.

What is good for checking leaks. I have no vacuum lines.

Thu May 22, 2014 7:25 am

Though not at all the safest proceedure, get a can of starter fluid and lightly spray around manifold and other areas of the carbs. If the idle goes up, you found the leak.

Thu May 22, 2014 7:53 am

jz56oval wrote: FreeBug wrote: The more i think of it, the more i'm thinking vacuum leak.

What is good for checking leaks. I have no vacuum lines.

Leaks can be at carb to manifold connection or manifold to head connection, or crack in manifold or lost plug on carb passage, vacuum is sneaky it can be anywhere and not always where you might expect it.......

Have you check the idle jets, are they really clean.... A clogged idle jet can cause you to have lean condition and can drive you nuts with the idle and low end responses ...

How good are your exhaust gaskets?

Yeah I know you did that all before.... Well do it again maybe you missed something.....

Dale

Thu May 22, 2014 7:58 am

Dale M. wrote: jz56oval wrote: FreeBug wrote: The more i think of it, the more i'm thinking vacuum leak.

What is good for checking leaks. I have no vacuum lines.

Leaks can be at carb to manifold connection or manifold to head connection, or crack in manifold or lost plug on carb passage, vacuum is sneaky it can be anywhere and not always where you might expect it.......

Have you check the idle jets, are they really clean.... A clogged idle jet can cause you to have lean condition and can drive you nuts with the idle and low end responses ...

How good are your exhaust gaskets?

Yeah I know you did that all before.... Well do it again maybe you missed something.....

Dale

I pulled the carbs out a weeks ago and checked the idle jets and cleaned them. Added all new gaskets to manifold to heads and carb o manifold. I didn't change the exhaust gaskets

Thu May 22, 2014 8:10 am

What gaskets are you using for the intake to the head? For sure don't use the metal ones. You need to check the base of the intake where it meets the head. I have found many times the base is not flat due to over tightening and needs to be resurfaced flat again.

Thu May 22, 2014 8:21 am

hooker wrote: What gaskets are you using for the intake to the head? For sure don't use the metal ones. You need to check the base of the intake where it meets the head. I have found many times the base is not flat due to over tightening and needs to be resurfaced flat again.

I'm using paper gaskets

Thu May 22, 2014 9:09 am

Well if you are completely satisfies the carb or vacuum leak is not issue, then its either timing or you electronic ignition going a little funky....

Dale

Thu May 22, 2014 9:40 am

surging idle is an intake leak.

Also, don't discount a leak between the 2 ports. Not leaking externally, but between ports will create some weird symptoms.

Thu May 22, 2014 11:20 am

You have to pull the carbs again to set the float height and drop so you may as well replace the gaskets again and clean out the jet circuits when the carbs are on your work bench. Also make sure that your manifolds or carbs are not hitting the shroud or tin or engine compartment causing the intake leak.

Pretty sure it is an intake leak-you can check spraying carb cleaner around the manifold when it is running and if RPMs pick up you found your leak, or use a vacuum line as a stethoscope, or spray some carb cleaner down each barrel when running and if RPMs pick up you located the barrel(either intake leak or idle jet circuit blocked).

Sun May 25, 2014 1:36 pm

So I replaced my spark plugs as they were black and #3&4 were even finger loose. Thought this was the problem. started the motor and it idled just the same as before. It idles around 750-800 and when I rev it up at park the RPM's drop down to 610 - 650, sounding like it is about to stall and then it goes back up to 780 RPM's. I sprayed carb cleaner around the manifolds at the head and there was no change. Also sprayed down the barrels of the carbs and they all bogged down.

Any other ideas on this problem? Im sure some one on here has had the same problem as Im having.

Wondering if I should take my carbs back out and thoroughly clean them. all the jets as well.

Sun May 25, 2014 4:36 pm

a return to idle and almost stall condition is indicative of an overly rich idle mixture.

Sun May 25, 2014 6:06 pm

What affect does turning the mixture screws in/out have?
If, as John has stated, the symptoms indicate extreme rich condition, if you turn in the screws can y get the idle to increase?

What procedure did you use to set best lean idle? After resetting carbs/gaskets did you reset the mixture?

Sun May 25, 2014 7:07 pm

Mikedrevguy wrote: What affect does turning the mixture screws in/out have?
If, as John has stated, the symptoms indicate extreme rich condition, if you turn in the screws can y get the idle to increase?

What procedure did you use to set best lean idle? After resetting carbs/gaskets did you reset the mixture?

When I turn in the mixture screws on all for, the idle slows or bogs. I just turn in or out on the screws to get the right idle. I adjust by sound.

I've also noticed that when I start the car it idles really low and takes a while to warm up. I have to Rev it at 800 or so for a while for it to stay started. Didn't do this before all these problems.