When I got home tonight I took both needle and seats out of the float bowls and blew them out. I then adjusted the floats and tightend down the carb on a new gasket.

I started it and it idled not too bad but when I went to drive it was chugging and did not run right at all. I know the motor is running perfect because I warmed it up with the old Holley 600.

I am fresh out of ideas as I don't think it's a vacuum leak or dirt in the needle and seat or float level adjustment. I wonder if I just got a bad carb, it's brand new but I know they are going to give me a hard time if I try and take it back.

Any more ideas ? :confused:

 

When you say chugging, is it running rich (black smoke/strong gas smell) or bogging like a lean condition.
If rich, have you checked the power valve.?
Do the idle mixture screws have any effect.?

I've never seen a 'bad' Holley, usually just a gasket or bad part.
However, without seeing it, anything is possible.

 

Warm the engine up then go out and make a really quick stop from 20 MPH or so. If the engine slows below idle or stalls your floats are still too high. Turn both floats down 1/4 turn and try it again. Repeat until the engine doesn't go below idle during a real quick stop. The problem may be that the floats are not set right when the under hood temps build up. Setting them when the engine is warm still doesn't account for the buildup of heat under a closed hood.
Got 'em set? Good.
Now check your vacuum at idle. If you have 10 inches of vacuum you are goijng to want a 7.5 or 8.5 power valve. You need for the power valve to be rated 2 inches of vacuum below what you have at idle. In drive for those with an automatic. Power valves all come in .5's so you'll have to experiment.
Now you can fiddle with the idle screws. Remember, both in or out the same amount.
Next up is the jets. You need to install freah plugs. Find a clear road and take your tools with you. Run the engine to about 3000 RPM and hold it for a little bit. Without idleing, pull over and watch the grass grow while your motor cools down enough to pull a couple of spak plugs. You should be looking for a tan color. If you have a life and don't have time for this you can go to your local performance tune up shop and have them tune the engine. They should have an exhaust gas analsyer that will give them the readings needed to correctly jet the car. Money well spent for most of us.

 

It chuggs like it getting no gas then runs then chuggs. The idle mixture screws seem to help.

This is the wierd part, when I first put it on it ran OK then it started to run lousy, I pulled over revved the motor a little and it backfired and then ran OK again.

I know this carb was a display model and had been sitting there for a while.
It also had a loose primary squirter that I tightened down. I also used teflon tape on the two fuel inlet lines because they were leaking. I can't see this causing the problem though.

Don't think theres any black smoke and don't smell gas. I adjusted the floats until gas started coming out of the inspection holes when running.


[Modified by MotorHead, 7:50 PM 4/16/2002]


[Modified by MotorHead, 8:40 PM 4/16/2002]

 

Floats are too high and you need to check your vacuum and get a matching power valve.

 

1979toy-->The power valve theory is very interesting however it did run perfect for a day. If it were the wrong power valve then it would never have run good at all ?

Thanks for your help it really sounds like you know how to tune a Holley.
One more thing I just tried it again and it actually runs like it has a plug wire off, but it dosen't.

Will check the vacuum tommorrow.

 

You could have pulled a vacuum line loose. That would drop the vacuum enough to make the power valve think that you were accelarating. Meaning it's going to dump extra fuel in to the motor. Don't ask how I know that one! You do have a line connected to the vacuum port at the lower right (pass. side) front below the float bowl don't you? Another don't ask item!
Good luck with it.

 

There is not much that actually goes wrong with a Holley. You have looked at or adjusted just about everything that could be the problem.

As 1979toy has mentioned, give a quick look at all the basics.
Vacuum lines, nuts/bolts etc...

I still don't trust the power valve. You did get a backfire once, however the newer Holleys are supposed to have power valve blowout protection.

Do you have access to another power valve just to give it a try ?
A 6.5 would be a good one to try. I think the 750 comes with an 8.5.
On mine, the 8.5 opened too early causing a bog. (Too rich too early)


[Modified by Barry's70LT1, 12:25 AM 4/17/2002]

 

I have been following this thread closely as my Holley is giving me fits. I do have a problem when decellerating the idle goes way down sometimes stalls if I really brake hard. My floats were set a little low so I brought them up to trickle out of the sight plugs. No improvement. Maybe they are to high! I took mine out yesterday and it ran great (6500 rpm blasts) no problem. After about 15 minutes of driving it started to chug and lose rpm. When I gave it some gas it finally cleared and ran decent but seems to be loading up when hot. I'll try the floats bringing them back down. How low is to low though.

 

If it were the power valve then it wouldn't have run good at one time ( I think )
And after the backfire it ran OK too for a while.

The only other thing it could be is dirt in one of the little passeges inside. I am probably going to take it apart and blow the whole thing out. Can you just buy the float bowl gaskets or do you have to buy the whole rebuild kit.

What do the guys who race doabout gaskets when they are playing with jets etc. Can't see buying new gaskets all the time.

 

I have a blue gasket that is reusable and I think it is made by holley or maybe Mr Gasket. You can buy them separately. I bought a quick change jet ket which has plugs in the front of the bowl which allows you too change jets without removing the bowl. Wish I hadn't bought it since the jet change wrench doesn't grip very well and I have had to remove the bowl almost everytime to retrieve the jet(s) that falls from the wrench. :mad

 

you say it back fired and then ran bad, is because you probably blew the power vavle when it back fired this is something that happens with holleys they dont like it when the engine back fires they take it very personal :yesnod: :lol: :cool: :smash: :seeya

 

You'r right about the power valve. Once it's bad, it should remain bad. However, it is the rubber diaphram in the power valve that fails. It is possible that a small hole in it might seal for a period of time.

Gaskets !! Yes you can buy just the bowl and/or metering block gaskets.
I get them from a local speed shop. I always carry a spare set at all times.

Beware,...... I have tried Mr. Gasket and other non Holley gaskets and they "WILL NOT LAST", and will cause problems in a short period of time.
Use genuine Holley parts, it's worth it.

General Comment....... If you have your fuel return line blocked, (the one going back to the tank), you will end up with all kinds of strange problems when the engine is hot.
I had mine blocked for awhile, and finally opened it again and solved some hot problems. (Starting and bogging)

Barry


[Modified by Barry's70LT1, 4:13 PM 4/17/2002]

 

What is this fuel return line you speak of? Pics?

 

blocking the return line will, almost double the pumps fuel pressure because the return line is a calabrated leak that the pump allows for. if your pump starts going bad on the road just clamp off the return line and you will get home. :chevy

 

Thanks again guys, one more question, can you visually tell if the power valve is blown I'd like to check it while I have it apart.

 

put it in your mouth and suck on the diaphram side and it should hold. :chevy

 

I'm afraid to ask hold what ? :lol:

Thanks for that info Clem, am just about to do that.

 

1979toy-
thanks for the instructions for tunning double pumpers-I copied it and am saving it for later.
Stupid question of the day----how do I adjust the floats?

 

Under the hood on the passenger side, on top of the frame, is where your gas line heads toward the fuel pump.
Beside the gas line is a smaller line running parallel with the gas line. This is the fuel return line.
On a '70 the small line is attached to the second outlet on the fuel pump.
I'm not sure if a '73 is the same.
Barry


[Modified by Barry's70LT1, 7:26 PM 4/17/2002]