gas leak? 7-8 mpg
 

Hey all, updated this original question below;

Got a problem I've been monitoring.
Problem is 7 miles per gallon. So, why such low mileage? I've heard 12-14 is usual.

First of all, its a stock 350 crate on 76 2wd frame with TH350. New edelbrock carb with phenolic spacer, intake and headers. Running good and shifting well. Not getting any flooding or smoke upon startup. More facts; only the passenger tank on it. all extraneous multi tank stuff is gone, sender is two outlet, one to the pump, other is vented along the bottom of the bed. I can't find a leak anywhere or anytime- sitting cold or while idling.
Pressurized the fuel supply by plugging the vent hose. Ran at idle for a few minutes, then checked for leaks while idling. Not a drop anywhere. Wiggled some fittings, moved hoses etc but I couldn't find a leak.The system, including tank, was well pressurized indicated by the small amount of gas that came out of the filler neck when I removed the gas cap while idling.

So, my next questions are; does my fuel pump diaphragm have a pin hole leak, passing fuel into the oil? And if so, any way to check that?
thanks in advance.

Re: gas leak?
 

anyone have any ideas?

Re: gas leak?
 

This is strange,

I had my 97 start doing the same thing last Saturday on the way to a show, It's a Demon with a Holley electric fuel pump near the tank with a hose venting at the frame as well. Had a guy pull next to me and yell I had a gas leak, I noticed the gauge was coming up faster than normal. I pulled over and couldn't find anything. At the show I smelled gas the whole time but still couldn't find a leak.

It will be Monday before I have time to jack it up so I can get fully under it and check it out closer, I plan on plugging this vent hose and carefully pressurizing the gas tank and see if I can find a leak.

Re: gas leak?
 

I think the charcoal canister should be hooked up, but I think the problem is
in the carb, the meatering rods may be stuck up in the WOT position, causing
it to run rich.
But I wouldn't rule out a fuel leak yet.

Edelbrocks seem to hurt MPG but I wouldn't think that much.

Re: gas leak?
 

restrorob, thats a good idea about plugging the vent and letting pressurize and then poke around for leaks. I'll try that.
heavychevy79, i have dropped the floats to 1 1/8 ", which seemed to help the black smoke upon warm start up. I don't get the smoke any more. In light of the possible metering rods stuck in up position, would that accentuate noticeable valve knock (at least it sounds light valves) when half pedal or more?

Additional question here then for anyone; does valve knock come from a rick or lean condition or is timing related?

Re: gas leak?
 

In most cases it's from the timing being advanced a little too high, But could be caused from running lean which could cause a little hotter than normal running temperature.

Re: gas leak?
 

Here's a update. Went to a speed shop the other day (austin performance). Guys there are very helpful and knowledgeable. Found out my distributor vacuum was on the manifold side. Bad choice on my part as I moved it from the recommended port to the manifold one. Needs to be above the butterflies on the carb. This was probably accounting for 2 mpg.
Found out distributor mechanical weights are likely sticking and springs could be worn out. New distributor is coming which improve firing and mpg with proper advance.
If oil is rising on the dipstick, you may have a leaking fuel pump. If it smells like gas - likewise. I had none of this. My issues appear to be timing and carb tuning.
FYI for those newbies out there like me.

Re: gas leak?
 

Glad to hear you may have it figured out. Keep us updated.

Re: gas leak?
 

Thanks- yeah, that's part of the reason I bought it. To understand how it works and be more self reliant and to have some fun.

Re: gas leak?
 

Another update;
Bought a TDC tool and checked my harmonic balancer #1 TDC mark against the timing cover TDC 0 mark. Found that my hb mark was about 14 degrees early. Now I am going to adjust the timing accordingly with a timing tape and new mark on the hb. This explains the lack of power I think and the knocking I heard as the mechanical advance kicked in over 1500 rpm. fyi.

Re: gas leak?
 

Did you change the fuel pump?

I can set timing by sound, but I still hate it when those dampners shift.

Hey is there some pictures of your truck around here?

Re: gas leak?
 

If the damper has in fact shifted, replace it!! That outer steel ring can do alot of damage should it exit the hub at 5 grand, been there!! Also, I was a Chevy mechanic for 33 years and I know for a fact the vac advance supply is from below the throttle plates not above. It needs manifold vac NOT ported vacuum. Find a port on the carb where full manifold vac is available at idle. Leave vac line off when you time it then reconnect line when timing is set correctly. My 84 gets 17 MPG with a Q-jet on a 350 if I'm real soft on the throttle. jim

Re: gas leak?
 

Some other threads about this.

http://camaroforums.com/forum/archiv...hp/t-3803.html

http://67-72chevytrucks.com/vboard/s....php?p=3274562

Re: gas leak?
 

I didn't change to electric pump yet. i plan to do that when I put a rear mounted tank in. It currently side mount passenger side.
I suspect the hb has slipped and I plan to replace it when I do the heads and cam this summer.
For now, I put the timing tape on and realized that i can't see what my total advance is because only 15% of the tape is visible as the water pump is in the way. Didn't notice this when using just a mark because the edge of the mark was visible. So, I either have the wrong balancer or wrong timing cover for the rebuilt block I bought. Could be a later block that requires a later hb- don't know what block I got when buying this rebuilt. I figured they would be close enough- maybe not.

That said, I changed the timing to correspond to the corrected hb mark and its definately snappy and responsive now. There is virtually no knock and I have my vac advance blocked off. I put in an MSD distributor afterwards and lighter springs for full advance earlier. It still seems like it need a pro's touch but maybe it will get better gas mileage. I'll let you all know.
No pictures for the moment. Just moved from TX to AB, Canada and my camera stuff is packed. I'll add some in the next month or so.

Re: gas leak?
 

hey id do the spark plugs now..
i pulled mine after noticing i have a timing issue and they were crap!
soo.. just a lil more ..
and! i woudl run some fuel treatment..
i use "seafoam" 1/3 in the oil 1/3 in the intake itsefd (slowly pour it in) and the rest into the gas...
iused to have a sticky vaulve due to storing my truck for over a year... a little bit of that.. and it went away during that oil change..
good luck dude..

Re: gas leak?
 

Thanks DM, I'll look into that.
another update;
couldn't fine tune the engine with my experience so took it the shop.

Tech moved the initial timing to 12 btdc ( i was as at 8). he hooked up my dist vacuum to the ported side and and adjusted my vac advance. This smoothed out my idle to 1800 rpm operation. He left the light springs on the mechanical advance (msd kit). Also found out my trans kickdown cable was being constricted by the edelbrock intake, 1 " phenolic spacer and carb setup. The kickdown is binding and a bit short limiting full throttle.
Looks like I may need a Lokar kickdown cable and a custom bracket to hold it. The Edelbrock throttle bracket is too low to the intake for the phenolic riser and carb causing some binding too. I was told Lokar will solve this. Need to look online.
FYI- for shadetree guys.

Lastly, I am using oil through the valve seals. I see whitish smoke with a hint of blue when its cold (32 F/ 0 C) in the mornings and no blue smoke. I can't believe this rebuilt would have seals crap out so quickly. I may just leap to Trick Flow heads now.

Re: gas leak?
 

WOW well i'll be honest and say that i would not have "guessed" half of those issues....
BTW i run my timing at 12 degrees (vac advance disconnected)
and i do not know the total timing because i dont have a full timing mark thingy
hahaha i said thingy!

Re: gas leak?
 

me neither. I would have thought that edelbrock would have engineered this possibility into their design given some of the high rise intakes they have. as I have it half Macgyver'd anyway, I guess I'll try lokar next. I might post a pic.

Re: gas leak?
 

Well here's something to consider for those ordering parts with a rebuilt engine. Went looking for the block casting number and Engine Code. Googled my codes and found that the core was cast in 1975. A few posts ago I noted that my harmonic balancer wasn't lining up with my timing cover marks on the tab when I used a TDC tool to find true TDC. It appears I have a timing cover from a later period than my core. This core type was cast from 68 to 79. I think the timing cover tab moved to the 12 oclock position from 80 on.

All that stated, my HB may have not slipped then. My TDC mark was around 3 oclock position but the tab was at 12. It was the wrong timing cover.
Thanks to O'rielly Auto parts for that.
FYI.