The Stovebolt Site Forums The Shop Area Engine Shop need carb for 270 gmc engine

Guys, I am a new to old trucks. I have a 1959 GMC w/ 270 cu engine. Had Carb rebuilt and just has issues I think it is just wore out. Would like to find a replacement carb for the truck but I am confused and have not been able to find anything. The carb on it now is a Zenith the tag has 234685 and 11963 for what ever that's worth. I understand from the manual that Rochester carb. were also used. I am not trying to be original just need something that is reliable. I can find a lot of carbs for 235's but cannot find a cross reference of a carb that would work on a 270. Any suggestions would be great.

"I have a 1959 GMC w/ 290 cu engine."
270 engine?

http://en.wikipedia.org/wiki/Chevrolet_straight-6_engine#GMC_engines

"I understand from the manual that Rochester carb. were also used."
Which manual/page?

I'd have the Zenith rebuilt.

The Zenith you have is one of the very finest carburetors ever to be built. They just don't wear out. Good for half a million miles, and can virtually always be rebuilt (of course, if someone drops it, or hits it with a hammer, it may become damaged).

With no offense meant, ignition issues are often diagnosed to be carburetor related.

Rochesters were never used on the 270.

Jon.

Thanks for the reply I will try to check out the ignition first no offense taken I am learning a lot and appreciate what steps or direction to take.

There are quite a few extremely talented individuals that read/post on this forum. If you describe your issues, someone here can probably help with where to start.

Jon.

Stay with the zenith, someone I trusted told me I could run a Rochester on my 55.2 gmc with a 248. They were wrong, and I was out $!

I had it rebuilt by a trusted mechanic. I work for a lumber yard and he has been the mechanic for 40 years here. He actually knows the lrean the trade working on thee year trucks. I do however realize the rebuild might not be good. But,it hesitates bad when I accelerate. It floods a lot. I am looking across from a new gas tank just received from LMC. The flooding I believe is rust from old tank plugging the carb. But the linkage is very loose. The throttle shaft(the shaft with the butterfly) has a lot of play in it. I now I can send it off for a more extensive rebuilt but I thought a new carb. would be better. The way it is now it is not reliable and the performance sucks. Some of the linkage I can tell has been bent and straighten. Any suggestions is great. I am trying to post pictures of the truck if I can figure out how. I rebuilt the truck myself by taking it completely apart piece by piece sandblasting the frame and working up.

LOL... While you're pointing out the generous help generally available at this forum Jon... *I* CLEARLY see three fingers pointing back at you.

As they should!
Helpfull, Patient, and FULL of Knowledge.

Now I guess I'll add Humble to that list.

OK, from your second post, looks like two issues: hesitation and flooding. Taking them in the order I would consider the most important.

The flooding can be caused by:

(1) defective or dirty fuel valve
(2) defective or incorrectly set float
(3) cracked housing where the fuel enters the carb (very unlikely)
(4) excessive fuel pressure (have you installed an electric pump?).
(5) blocked or missing fuel tank vent (can creat too much pressure if too close to the exhaust)

Always install a new fuel filter after the carb has been rebuilt to protect the work done.

Hesitation:

Yes, hesitation can be caused by a defective accelerator pump in the carburetor, but highly unlikely in a Zenith with a fresh rebuild.

Much more likely to be in the distributor. If that model has a vacuum advance, is it working? If not are the distributor plates free (not sticking) and thus allowing distributor advance?

Now as to the linkage being loose:

Play in the throttle shaft IN THE DIRECTION OF THE SHAFT IS NORMAL.

There should be some play (or clearance) between the shaft and the throttle body. If there were none (or too little clearance), then the shaft would bind in the throttle body when the engine was hot, due to the coefficient of linear expansion.

The design tolerance of shaft to body (the direction perpendicular to the shaft) when new was 0.004~0.006 inch. Wear in the field as much as an additional 0.003 (so a total of 0.009) inch is acceptable. This is easy to measure with a dial indicator.

More than 0.009 can cause idle issues; but may be repaired either by installing throttle bushings or a new throttle shaft or both depending on the location of the wear.

Jon.

A Zenith model 28AV11 would do nice. Cheap to rebuild.. NOS rebuild kits are $15-20 on ebay. Suckers pay $50+ for a kit.