Motor bogs down @ 4K RPM

[WhiteKnight]

The motor is a '71 L-48 with auto tranny. If I leave it in drive it will shift around 4000 RPM 1-2 shift. With the selector in 1st it will bog a little over 4k then it almost sound like its missing but makes it to 5300 and then shifts to second. If I manually shift to second when it first bogs all is fine until around 4500 in second. I thought it might be ignition related and since I was repalcing the distributor (HEI unit no mech tach drive) with a points unit w/mech tach drive (thanks Jack ) I also replaced the plugs, wires and tach cable, added a coil and balist resistor. I can now see the RPM at which this happens rather than calculate it based on speed, tire diameter and gear ratio. However, the problem remains. I thought it might be fuel starvation so I replaced the fuel filter and also found the accelerator linkage not adjusted to open the secondaries fully at WOT. I made the adjustment and that gave me 200-300 more RPM but still the bog happens. Note the carb is from a '74 454 (Carb Rochester Q-Jet: 7043201 = (1974 454ci 275hp man trans). Not sure if that matters, it has a mechanical choke and it seems to operate properly (closes when cold with the first pump of the peddal and opens when warm).

Compression test #1, #3, #7 - 130, #2 & #4 - 135, #5, #6, #8 - 140.

The old plugs # 1, # 3 & #7 had carbon deposits. The rest were normal.

When first starting it will smoke (blue) a little from the drivers side, but if I shut it off and re-start, no smoke.

Revving the motor with no load, it revs fine all the way to redline, yes I only have done that after warming up the motor. No miss.

What next? I'm thinking either something in the trans is messed up.. or maybe it's the carb, but since my knowledge is from reading more than doing lately. The last time I rebuilt a motor was in high school auto shop 22 years back. I need advice.

[Carnalsupply]

Hi Knight,

I'm just another backyard wrencher like a lot of us here, but it sounds much more like a carb problem than an ignition one to me. Why? The change in RPM when the trans shifts will do little or nothing to affect ignition, save for tickling the vacuum advance, but by this RPM I would have to think the mechanical advance would have complete control of the ignition, and besides, you changed distributors and the problem persists.

I would, if at all possible, borrow and swap carbs as a simple and quick test. The '74 vintage Rochester you're using wasn't much of a performer when new, and now it's some 30 years later which doesn't bode well for it's ability to correctly meter air/fuel.
If you can borrow a carb of around 600-650 cfm and swap it in, it would be interesting to hear if the problem continued. If you try this, try and use a carb in good running condition, not something somebody pulled off a shelf after a few years. I would try and find a Holley and stay away from the Q-Jets, simply because the Holley is much more known commodity, hence easier to tune and find parts for.

Like I said, this is just my opinion, but it's where I'd start.

Best of luck.

[GTR1999]

Check to see if you have the correct "S" fuel hose from the pump to the main line from the tank. If you have plain fuel hose there instead of the "S" hose it might be kinked or compressing under load. I had the same thins happen to me with my 72 auto years ago. Replaced the hoses with the correct ones and no fuel starvation.
Gary

[Jack]

I thought it might be ignition related and since I was repalcing the distributor (HEI unit no mech tach drive) with a points unit w/mech tach drive (thanks Jack ) I also replaced the plugs, wires and tach cable, added a coil and balist resistor. I can now see the RPM at which this happens .

Seems someone & their vette found a play-purdy under the tree. That distributor's a good piece ... I checked, tightened & lubed it all up for Mrs. Claus.
Happy New Year
JACK

[grapeknutz]

The motor is a '71 L-48 with auto tranny. If I leave it in drive it will shift around 4000 RPM 1-2 shift. With the selector in 1st it will bog a little over 4k then it almost sound like its missing but makes it to 5300 and then shifts to second. If I manually shift to second when it first bogs all is fine until around 4500 in second. I thought it might be ignition related and since I was repalcing the distributor (HEI unit no mech tach drive) with a points unit w/mech tach drive (thanks Jack ) I also replaced the plugs, wires and tach cable, added a coil and balist resistor. I can now see the RPM at which this happens rather than calculate it based on speed, tire diameter and gear ratio. However, the problem remains. I thought it might be fuel starvation so I replaced the fuel filter and also found the accelerator linkage not adjusted to open the secondaries fully at WOT. I made the adjustment and that gave me 200-300 more RPM but still the bog happens. Note the carb is from a '74 454 (Carb Rochester Q-Jet: 7043201 = (1974 454ci 275hp man trans). Not sure if that matters, it has a mechanical choke and it seems to operate properly (closes when cold with the first pump of the peddal and opens when warm).

Compression test #1, #3, #7 - 130, #2 & #4 - 135, #5, #6, #8 - 140.

The old plugs # 1, # 3 & #7 had carbon deposits. The rest were normal.

Oil deposits on plugs

When first starting it will smoke (blue) a little from the drivers side, but if I shut it off and re-start, no smoke.

Valve seals are bad. Thats what you see on the plugs!

Revving the motor with no load, it revs fine all the way to redline, yes I only have done that after warming up the motor. No miss.

What next? I'm thinking either something in the trans is messed up.. or maybe it's the carb, but since my knowledge is from reading more than doing lately. The last time I rebuilt a motor was in high school auto shop 26 years back. I need advice.

Most likely its the carbs fuel level is not right. The carb needs to be most likely rebuilt. The Q jet is a really a great carb, but is misunderstood on how it works. Find a good carb rebuilder ( not auto zone et al ) and have it done right If you can afford it check out Barry Grants site.

[lightingrod]

Whiteknight,

It's really hard to give sound advice with just the info you provided! Is it missing or is the power just falling off? The fact it's still happening with all the changes you have made does pretty much take a few things out of the mix!

Here's what I would do and the order I would do them!

1. Check the timing! Check Top Dead Center (TDC), with an older engine like ours you never know whats been changed, worn or broken! The damper or pointer may be off!

2. Check the Mechanical and Vacuum advance.

3. Pick up a set of new plugs, find a lonely stretch of road with no traffic. Install a plug and do a full speed blast, if safe I would put it in "D" and run it through all 3 gears. Kill the ignition, DON'T LOCK IT! Also know, no power steering or brakes! Pull off the side of the road and pull the plug! If it's running rich enough to cause your problem it should have some black residue. If there's not, it does not mean your carb is spot on by any means, it just means that it's most likely not way way off!

Hope this helps!

Al

[WhiteKnight]

Thanks for all the replys.

Looks like everyone is thinking fuel related problem. So I'll start there.

GTR1999 - the hose at the fuel tank does not look like it has a S curve.. posting a picture.

Jack - the distributor works great!! Thanks!

GrapeKnutz - I'll ask some of the local vett guys who they recommend on the rebuild.. I have been thinking of getting a rebuilt Q-Jet from Carbs Unlimited (correct numbers and date code as the numbers and casting codes match on 98% of the car rear diff, carb, alternator and distributor non matching). Anyone dealt with them? http://www.carburetion.com/quadrajet.asp

Lightningrod -

1. I followed the article from the CAC knowledgebase when instaling the distributor: http://www.corvetteactioncenter.com/....php?qstId=440
Once TDC was found the damper mark was spot on zero of the pointer.

2. The distributor is an Accel model# 20100T dual points mechanical advance only. I set the points gap @ .019, then the dwell on a single set to 26' and then 34' with the second set added. Then set the timing at 8 before TDC. Then checked it at 3000 rpm and it was 38'.

3. Sorry, I intended to mention that I pulled the new plugs after my first run. They looked fine except #1, #3 & #7 are showing oil is getting on them.

How difficult is it to re-build a carb?

[WhiteKnight]

Just made a pizza run and it is not a miss more like a power loss. Like hitting a wall.

Here is the picture of the fuel hose at the tank.

[lightingrod]

WhiteKnight,

I'm not ready to blame it on fuel yet! If I was, I would guess floats or fuel pressure/volume! A pressure/volume test? The curve in the fuel line, not! When you installed the new plugs, did you do a single WOT pass and then check them or just a bit of driving? If your showing oil after one high speed blast, you got other problems!

Your trouble shooting steps so far seem sound, hang in there and take you time and keep us informed, that's the way we all learn!

Can you get a carb fom a buddy, wish you were near I have a new Edelbrock 750 cfm just sitting here!

Al

Al

[speedmaster4]

blue puff on cold start almost sure oil getting past valve seals..old seals hardened and leak...worn valve guides can make seals job impossible...if it smokes going downhill with throttle closed--valve seals or worn piston rings,blowing heavily out oil fill hole at idle its rings.

cut-out at higher rpm does sound like fuel prob...tee into fuel line just before carb and run hose out to gage so you (or helper-better) can watch it while driving...if it drops below 4 psi its bad...

[WhiteKnight]

Lightingrod,

Immediatly after I got in in tune I drove it out of the neighborhood where I have a nice 3 mile long stretch of road with little to no traffic and no cross streets. I intended to give it a nice hard run an pull the plug to check the heat range. Which I did do but couldn't really tell anything from it, except the plug I pulled, #5, looked about like it did when I put it in. I took it back home and pulled all the plugs on the drivers side. As I said before all but #5 looked a little oily. Not like the old ones I pulled out, but not as clean as #5.


The motor has burned a quart per tank since I had it. (Sept 30th, 2005). I spoke with the previous owner and they had not done any motor work on it. I got it from a vette dealer in Atlanta, but the title was from NC and still in thier name. They had had it since 1990 and the OD showed 52K to which they added 4500 more miles in that time. I've added another 2K, half of which was the trip home. The car shows it too.. looks good but lots of seals are starting to leak. The bog problem started in November. A week or so after I had re-connected the fuel vapor recovery canister, at the time I thought it might be the problem and dis-connected it and put the single connection PCV valve back in but still had the bog.

I have noticed a strong fuel odor in the garage but have found no obvious leaks.

I'll work on getting a test carb.. hmm Lone73 is preping his '73 for paint.. Hey David, buddy can ya spare the carb for an afternoon??

[WhiteKnight]

Speedmaster4 -
I am not positive the motor has not been pulled, but since it is a '71 and was built to use the leadded fuels available then it probably has not had the heads worked on for todays unleaded, if at all. Can't remember exactly what needs to be done but have seen GMJunkie post about it.

I may end up trying that "T" idea, but it currently has the hard line from the pump to the carb.

[black_81_vette]

Did you check the fuel line by the sending unit? could be rotted ect and sucking together.Only other possibility is a cam shaft is going flat and or you have a broken valve spring.mine ran the same way and I had a lobe going flat on a exhaust lobe.

[Jack]

I am not positive the motor has not been pulled, but since it is a '71 and was built to use the leadded fuels available then it probably has not had the heads worked on for todays unleaded, if at all. .

Don't hold me to it ... but it's my understanding that MY1971 sbc heads DO have hardened valve seat ... this feature in preparation for known upcoming unleaded fuel requirements ... even though cats did not appear until MY1975. Again, I'm not CERTAIN ... only my understanding. Welcome others to chime in w/ additional info? Dunno if you have excessively-worn guides or not ... maybe you do ... but that has nothing to do with pre-lead/post-lead seats ... guides are guides, seats are seats.
JACK

[speedmaster4]

your heads should have hardened seats..like Jack sez,not part of this problem...given your relatively low mileage, valve guides are probably not shot,but time likelyhas taken toll on seals--they harden and leak...fortunately you can change the seals without pulling the heads--i'd do that

i strongly suggest u cut into your fuel line to isolate your problem...use a tubing cutter--do not hacksaw--then some fuel hose,clamps,tee,and gage and you will know whether carb is bad or supply problem...after test you can rejoin metal line with hose or compression union....but if you prefer to play "musical" carbs, i really do hope you hit the head on the nail