Fuel System Help Needed

[Roy L.]

I have a 69 with a 427 390hp that I purchased last fall. I had it out a few times over the fall and winter and with no problems - car runs very strong. I've had a problem show up since its gotten warmer and the car is being driven more.

The car runs fine as long as I dont shut it off. If I try to restart it while the engine is hot, the carburetor floods and dumps gas onto the intake manifold. I've also experienced problems with idle when sitting down hill - although I cant say whether that happens only when the engine is hot or not. It is the original Rochester 4MV carb and original intake.

I've replaced the carb/intake gasket and heat shield. I found the gas cap was not venting and have replaced that. I've also checked that the fuel return line to the gas tank is clear.

Does anybody have any experience with something like this, or have suggestions on the next step? I suspect the problem is with the carburetor, but I will be the first to admit I have no skills with a carburetor.

[Edward J.]

Re: Fuel System Help Needed

Roy, sure sounds like a float level that maybe to high, or a heavy float.

[Gene M.]

Re: Fuel System Help Needed

Roy, sure sounds like a float level that maybe to high, or a heavy float.

In addition to the above I suggest you replace the inlet seats. It sounds like they have deteriorated.

[Joe L.]

Re: Fuel System Help Needed

Roy, sure sounds like a float level that maybe to high, or a heavy float.

Roy and Edward------


Yes, the first thing I'd check is the float. Rochester Q-Jets use a Nitrophyl (plastic) float. The Nitrophyl is a very light, foam plastic which is coated with a "gasoline-impervious" clear coating. Over time, the coating breaks down and the plastic absorbs gasoline.

I'd rebuild the carb and install a new float. The floats are not supplied with a rebuild kit (although they should be), so you have to purchase it separately.

[Roy L.]

Re: Fuel System Help Needed

Thanks for the info. I'm going to have a local shop that has a good reputation for rebuilding older carbs do the work for me. I noticed in the parts catalogs carburetor mounting kits that include two bolts and two studs. My carb is mounted with 4 bolts (2 long - front, 2 short - rear). We're studs originally used for the rear of the carb mount?

[Joe L.]

Re: Fuel System Help Needed

Thanks for the info. I'm going to have a local shop that has a good reputation for rebuilding older carbs do the work for me. I noticed in the parts catalogs carburetor mounting kits that include two bolts and two studs. My carb is mounted with 4 bolts (2 long - front, 2 short - rear). We're studs originally used for the rear of the carb mount?

Roy------

Studs were used for the rear mounting points.

[Jack H.]

Re: Fuel System Help Needed

I went through two original floats on my '71 with each lasting about two years. My gut tells me the combination of alcohol doped in today's pump gas with long winter periods of disuse (fuel in the bowl evaporates leaving the float exposed to air) is 'hard' on the factory original design/construction of the float...

So, the last time around, I installed an aftermarket (Filko, I think) hollow brass float that the mfgr listed as being 'correct' for my QJet by PN. It's performed without a hiccup for +7 years now and better than 40K miles.

[Roy L.]

Re: Fuel System Help Needed

Roy------

Studs were used for the rear mounting points.

The only torque reference I've found is in the AIM, stating 10-15ftlb. I assume that is for the fastener torque. Is it the same for the stud installation or higher? Should the threads be treated with anything before installation?

[Roy L.]

Re: Fuel System Help Needed

I went through two original floats on my '71 with each lasting about two years. My gut tells me the combination of alcohol doped in today's pump gas with long winter periods of disuse (fuel in the bowl evaporates leaving the float exposed to air) is 'hard' on the factory original design/construction of the float...

So, the last time around, I installed an aftermarket (Filko, I think) hollow brass float that the mfgr listed as being 'correct' for my QJet by PN. It's performed without a hiccup for +7 years now and better than 40K miles.

Thanks. I'll do some looking and see if I can find them. It sounds like a good investment in time.

[Joe L.]

Re: Fuel System Help Needed

Thanks. I'll do some looking and see if I can find them. It sounds like a good investment in time.

Roy------


Unfortunately, I believe that Filko is out-of-business now. However, you can get the floats from NAPA. The brass replacement for your application is their number CRB-2467. The stock, nitrophyl float is CRB-2440. Either one is about 10 bucks.

The floats are no longer available from GM.

[Roy L.]

Re: Fuel System Help Needed

The shop that is rebuilding the carb for me found a major problem with one of the floats - it had apparently swelled up. The rebuild is about finished. The throttle shaft bushings have some wear and may cause some minor problems with consistent idle speed - but cant find a source for them. Does anybody know of a source? This is for the Rochester Q-Jet.

[Joe L.]

Re: Fuel System Help Needed

The shop that is rebuilding the carb for me found a major problem with one of the floats - it had apparently swelled up. The rebuild is about finished. The throttle shaft bushings have some wear and may cause some minor problems with consistent idle speed - but cant find a source for them. Does anybody know of a source? This is for the Rochester Q-Jet.

Roy------

I don't think Q-Jets originally used bushings on the throttle plate shafts. The shafts were simply installed in holes drilled in the throttle body. So, if it has bushings, they must have been previously installed as part of a repair.

GM never sold bushings to repair throttle body shaft bores but, once-upon-a-time they would sell you a complete throttle body assembly. The bushings are an aftermarket-type repair item. However, I would think that any good carburetor repair shop would have them.

[Timothy B.]

Re: Fuel System Help Needed

I can only speak to Holleys but pay attention to the base plate.

The base is a source of many problems from sticky throttle blades to wore primary shaft bores to warping where idle vacuum finds gas from the transfer slot plumbing.

[Roy L.]

Re: Fuel System Help Needed

Roy------

I don't think Q-Jets originally used bushings on the throttle plate shafts. The shafts were simply installed in holes drilled in the throttle body. So, if it has bushings, they must have been previously installed as part of a repair.

GM never sold bushings to repair throttle body shaft bores but, once-upon-a-time they would sell you a complete throttle body assembly. The bushings are an aftermarket-type repair item. However, I would think that any good carburetor repair shop would have them.

Thanks for the info. I checked to see if I had any photos that show that shaft, but I do not. As I recall, it appeared there was a bushing about 3/8" that the shaft went through - although that may have just been a line made in the base manufacturing.

The mechanic told me that he used to get the bushings with no problem, but it's been some time since he's worked on one of these and they are no longer available from his old sources.

I think I may have it reassembled as is, and find out whether it is a serious enough problem to do something about.

[Roy L.]

Re: Fuel System Help Needed

I can only speak to Holleys but pay attention to the base plate.

The base is a source of many problems from sticky throttle blades to wore primary shaft bores to warping where idle vacuum finds gas from the transfer slot plumbing.

Thanks for the tip. I've got a couple of options for checking the flatness of the base plate. I will do that before I reinstall.