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Questions for you LT-1 owners with stock engines. What is your manifold vacuum at idle? What is your initial timing? and does your rpm increase when the distributor advance is re-connected?
My 72 LT-1 pulls around 13-14 Hg at 750 RPM and I am running 12 degrees of initial timing. My idle definitely increases when the vacuum canister is connected. I’m running a B28 canister. My idle speed with the vacuum canister connected is 900 RPM.
Thank you. Just what I needed to know. I'm pulling 10 hg. Initial timing at 12. No increase when vacuum connected. Not sure what's going on here.
Do you have the distributor vacuum advance connected to ported, no vacuum at idle, or manifold, vacuum at idle and at cruising rpm?
The other factor when asking about vacuum at idle is what RPM, you will get less vacuum the lower the RPM.
So one car having 10 and another having more is meaningless unless it's the same speed.
I have a 70 LT1 short block assembly and find the best idle is 900 RPM.
Thanks for your reply. I've checked it with vacuum advance connected to manifold vacuum, not ported. Idle RPM 800 to 1000 and vacuum reading remained around 10 hg. The original issue was a rich running condition. Found several problems with the carburetor. Idle adjustment screws still have no effect but I'm concerned about the low vacuum reading.
Thanks for your reply. I've checked it with vacuum advance connected to manifold vacuum, not ported. Idle RPM 800 to 1000 and vacuum reading remained around 10 hg. The original issue was a rich running condition. Found several problems with the carburetor. Idle adjustment screws still have no effect but I'm concerned about the low vacuum reading.
Did you replace the power valves to correct the idle mixture screw having no effect? If the diaphragm blew it will leak gasoline.
It also sounds like the vacuum advance diaphragm is blown.
I guess there could be a vacuum leak at the manifold. I've disconnected all the vacuum accessories from the carb and there is no change. I also thought if there was a vacuum leak I would have a lean burn condition, not rich. I don't know any previous history on this engine. I'm wondering if a problem in the valve train can cause the low vacuum I am seeing. It's a steady reading but I was not familiar with what an LT-1 vacuum should be. Think I'll walk away from it for a while and work on other projects. Thanks to all for your replies.........................UJ
Yes, I replaced the power valves and have tested and re-tested them. I also made sure that the transfer slots were not too exposed under the throttle plate. I've also checked the vacuum advance with a Mityvac.
Questions for you LT-1 owners with stock engines. What is your manifold vacuum at idle? What is your initial timing? and does your rpm increase when the distributor advance is re-connected?
Most timely question as I was reinstalling the as purchased QJet carb on mine (relatively new to me) last weekend
I believe I have a factory replacement block, still solid lifter correct heads and unknown but non stock cam with single plane cast iron intake manny and the aforementioned QJet.
I was concerned with the low vacuum and the very bouncy readings and chalked it up to an aftermarket cam. Seems like that is standard for one of these?
Your question reminds me that I did not disco the the distributer advance. Also did not have my timing light but dug it up and
I'll rerun this weekend when I have a minute.
Yes, I replaced the power valves and have tested and re-tested them. I also made sure that the transfer slots were not too exposed under the throttle plate. I've also checked the vacuum advance with a Mityvac.
Did you check the secondary throttle stop screw. Should be roughly 1/4 to 1/2 turn from fully closed.
Did the distributor points plate move when you used the Mityvac?
Does your timing advance when you connect vacuum to distributor?
Have you ever adjusted the valves, too tight, low vacuum.
I did look at the secondary stop screw but wasn't sure were to set it. I will take another look. Thanks.
Yes, distributor plate moved when using Mityvac. But I will also take another look at that.
Timing does not advance when connecting distributor to manifold vacuum.
Don't know history of engine so don't know how valves were adjusted. Are you saying the low vacuum can be caused by valve lash being set too tight?
If the plate moves when connected to Mittyvac but doesn't show timing increase when connected to manifold something is not right.
With the engine running, if you place your finger over the connection on the manifold where the distributor vacuum advance is connected, do you feel any kind of suction? Even 10" should cause some advance.
Correct, tight valves, low vacuum.
Google for the John Hinckley paper on 30-30 valve adjustment. It includes a section on LT1 .
I will check the vacuum at the port where the advance connects for suction again to be sure. Maybe the vac can is not reacting as it should, or maybe incorrect vac can. It did seem kinda sluggish when I checked it with the Mityvac.
I am familiar with John Hinckley's paper. Thanks again. You have helped reinforce my thinking.
Thanks for your reply. I've checked it with vacuum advance connected to manifold vacuum, not ported. Idle RPM 800 to 1000 and vacuum reading remained around 10 hg. The original issue was a rich running condition. Found several problems with the carburetor. Idle adjustment screws still have no effect but I'm concerned about the low vacuum reading.
I am curious what you meant by "several problems".
And did you mean idle mixture screws have no effect?
I had several problems with my original LT-1 carb also.
A close-up photo of the choke horn where the numbers are and the flat pad in front of that, and the idle air bleeds may hold some clues.
IIRC mine idled around 10.5" vac with the 30-30 camshaft, not the original LT-1 cam.
The main problem with the carb was a leak in the secondaries and fuel working past the needle/seat o-rings of both bowls. Fast idle cam was also sticking. Yes, I meant the idle mixture screws; thanks for noting that. The carb is the correct 4150 dual feed for a 1971 LT-1 application. List 4801 and correct numbers on the metering blocks. The air bleeds are clear and do not look like they have been altered. I am familiar with the 3030 cam but not with the LT-1 cam, which is why I was asking what the vacuum should be at idle. I figured they would be about the same. I haven't looked at my '64 L76 in a while but I think the vacuum at idle was in the 12 to 14 range. I'll check it next time I fire up the L76.
Well I was concerned that you mentioned the idle mixture screws had no effect.
The leaking fuel needle/seat could easily cause that.
Try to tune the mixture to highest vac with a vac gauge.
If it the idle mixture screws still do not work I have one more place you can look for "problems" that is unique to this "emission" era OEM Holley.
I will see if I can find a pic or a diagram.
I would sure appreciate one more place to look at. The mixture screws had no effect last time I ran it. This was after I corrected the leak past the needle/seat. I have the carb off the car now to take another look at it on the bench. I also made some progress with the vac advance problem. I'm embarrassed to say that the hose I was using was poorly spliced causing a leak. All is good now.
Ok if you have done all the other common checks, and are still have mixture screw problems, so did I. One side of mine responded drastically different than the other to the mixture screw. It made no sense.
Until...
Refer to the pic below: This is my 4803 Holley for my 454 LS6. It is an almost identical twin carb to the LT-1 carb, minus some jetting changes for the extra cubes. However this one is not a factory installed carb. This one was a purchased over the chevy counter version mfg'd in the mid 70s. You want to pay very close attention to the "dimple" area outside the choke horn circled in blue.
Here is the same dimple looking straight down from above the carb. That dimple is now circled in red. Notice how my carb has a tapered metallic cone shaped bottom in that dimple. 99% of Holleys look like this. A factory OEM LT-1 carb is not like that. My 1970 LT-1 carb had what looked like gold colored body putty in that indentation area. And because of several carb cleanings it had started to come loose and crack. I removed it with a very small pic tool and found a SCREW underneath!?!?!? WTH!?!?! That factory carb had adjustable air bleeds installed to help the solid lifter / Holley motors get thru 1970s emissions. I did find a reference to it in a Holley book once. The screw looked much like an idle mixture screw but was smaller, brass, and had a ROUND tip, since it was for air. If you look directly across the choke horn in the pic above you will see the corresponding air bleed (blue). IIRC this is the low speed air bleed. You can see the brass restriction placed in my 454 "counter" carb above, because that one does not have an adjustable air bleed, it has a fixed orifice brass air bleed, although pretty large. A true factory installed LT-1 carb will have NO brass restriction in that air bleed, because the restriction is done by the screw. The above air bleed is a large fixed orifice because it is for a 454. Over the counter 350 carbs have smaller orifice air bleeds. But my factory LT-1 carb had none at all, just a big gaping hole! And One of those screws had become loose, and had changed it's adjustment, and was different from the other side. That is what cause my idle / low speed problems. I basically had two different air bleed settings right side & left side!
I sent it back to Holley in the early 1980s and they rebuilt the carb in their hot rod shop. They even looked up the factory blue-prints and found the part number for that screw and replaced both of mine. (It was a brass screw and I had ruined the slot on one.) They tuned the air bleeds on their flowbench to factory specs. Even the Holley tech did not understand what I was talking about until he saw the carb, and found the original blue-prints. Even in the 1980s no one at their shop remembered ever seeing one of those carbs with those screws before. It is easy to see how they would get overlooked in even a good rebuild / refurnish. They may be very rare, and only found in '70-72 OEM solid lifter Chevy Holleys. So 70-72 LS6 & LT-1 only. I am pretty sure the 69s did not have these screws.
So check your carb to see if you have the missing brass restriction in the air bleed, and if your "dimple" looks different than mine above. Then you will know if you have the hidden air screws or not, just with that visual inspection. If all other tuning tips fail, talk to Holley! If Holley can't help you anymore, (it has been 40 years) I would think you would have to adjust them on a dyno by tuning the AFR with an oxygen sensor. After you find someone that understands how to adjust air bleeds!
They made more Chevelles with that carb (4500 LS6s) or Camaros (4500 LT-1s) so you may find more knowledge over there, than here.
Good luck!
I just looked online quick and found an 081 dated 4801 Holley. The pic just so happens to show the missing brass restrictor in the low speed air bleed inside the choke horn! So this must really be an OEM factory installed carb. This pic unfortunately does not show a close-up of the "dimple" area.
Here is another 4801 I found online that shows the drilled "dimple" location that contains the hidden screw. This looks original to me also. But it also looks like someone "picked" at it. I am not sure if that is the screw or putty we are seeing there. Since the last carb pic'd is priced at $1495, and looks beautiful, you would think it would work perfect! But I doubt it. Those screws are a PITA. I still have tuning nightmares over those things.
Last edited by leigh1322; Mar 17, 2022 at 09:22 PM.
1971 LT-1 vacuum
