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Today I helped my stepdad work on his C3, as it needed to be tuned up and wasn't making any power since he bought it a while back. It's a really nice '78 that someone did a nice restoration on previously. It has a 305HO in it (we don't know why) , with a Weiand intake, Holley Street avenger 670, headers, and magna flow mufflers. There is what sounds like a BIG cam in it, it has a really LOPEY idle. Car has a 700R4 swapped in it too.
First thing we did was set the timing, which my assumption was it would be way off: it was around 18 degrees all in! I set it at 34 total and that gave it 20 initial, which helped the running and idle. We went and drove it: Still a PIG!
While driving it we thought it felt like the secondaries wouldn't open. Came back home and revved it, saw they wouldn't open. Checked them for operation, and they were good. So put in the lightest spring for the secondaries, and went and drove it: no change.
It doesn't smell like it's rich, but won't know if the fueling it good until we put the wideband on next and tune the carb. When you shift from idle to gear, there's a big drop in RPM (from 1000 to 600)... This makes me think this big cam is trying to fight a stock torque converter!
So we have a car that feels like it makes 100HP, won't rev past 3500RPM, and is a total PIG. I'm scared it's way over cammed, stock converter, and highway gears. I don't have the specs on anything, so no way to confirm. If this is the case, wouldn't I still be able to rev the engine up to the 5500 redline?
Next steps are wide band to ensure the carb is tuned correctly, what else could be causing this massive loss of power? What should we work through next? It runs great at idle and low RPM cruise, but when you step on it nothing happens lol
The drop in rpm from park to drive is most likely caused by your vac advance.
check park vacuum versus drive vacuum, I bet there’s a noticeable difference.
Which in turns pulls timing, compounding the problem.
you could test it by just disconnecting the vac advance and seeing if that resolves the rpm drop. If it does, then the solution would be a vac adv can that’s “all in” at a lower vacuum
If it is a realtively low CR and a too big duration cam you could likely have horrible cylinder pressure.
Check the compression and let us know how bad it is. If it's really bad that could be your whole problem.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by MelWff
Still curious about the 305 HO part.
305 HO was the L69 built from 1983 to 1988 and used in the Monte Carlo SS, Trans Am and Z-28. It was actually a good running performance package, and could have been easily installed by someone into a Vette. It had a computer-controlled Q-Jet, so if someone put a 305 HO into a '78 Vette without the computer it's not going to run very well... It also had a computer controlled distributor, so if the OP was able to set total and initial timing it sounds like someone has replaced the distributor. If that's a 305 HO, it means that the OP has a real mix-n-match hack-job can of worms engine on his hands...
The drop in rpm from park to drive is most likely caused by your vac advance.
check park vacuum versus drive vacuum, I bet there’s a noticeable difference.
Which in turns pulls timing, compounding the problem.
you could test it by just disconnecting the vac advance and seeing if that resolves the rpm drop. If it does, then the solution would be a vac adv can that’s “all in” at a lower vacuum
Good insight, this slipped my mind because everything I own has a manual!
If it is a realtively low CR and a too big duration cam you could likely have horrible cylinder pressure.
Check the compression and let us know how bad it is. If it's really bad that could be your whole problem.
Yeah I'm scared of this and it's a possibility - next step needs to be a compression test.
305 HO was the L69 built from 1983 to 1988 and used in the Monte Carlo SS, Trans Am and Z-28. It was actually a good running performance package, and could have been easily installed by someone into a Vette. It had a computer-controlled Q-Jet, so if someone put a 305 HO into a '78 Vette without the computer it's not going to run very well... It also had a computer controlled distributor, so if the OP was able to set total and initial timing it sounds like someone has replaced the distributor. If that's a 305 HO, it means that the OP has a real mix-n-match hack-job can of worms engine on his hands...
This is correct, for some reason, someone spent a bunch of money on this car and built a 305 HO! It has the 416 heads, a big cam, a Weiand intake, headers, and a 670 Street Avenger Holley. We don't know the specs on any of it, but it runs horribly! It's my step dad's car, who taught me everything I know about wrenching so, i've got to get it sorted for him.
Today I helped my stepdad work on his C3, as it needed to be tuned up and wasn't making any power since he bought it a while back. It's a really nice '78 that someone did a nice restoration on previously. It has a 305HO in it (we don't know why) , with a Weiand intake, Holley Street avenger 670, headers, and magna flow mufflers. There is what sounds like a BIG cam in it, it has a really LOPEY idle. Car has a 700R4 swapped in it too.
First thing we did was set the timing, which my assumption was it would be way off: it was around 18 degrees all in! I set it at 34 total and that gave it 20 initial, which helped the running and idle. We went and drove it: Still a PIG!
While driving it we thought it felt like the secondaries wouldn't open. Came back home and revved it, saw they wouldn't open. Checked them for operation, and they were good. So put in the lightest spring for the secondaries, and went and drove it: no change.
It doesn't smell like it's rich, but won't know if the fueling it good until we put the wideband on next and tune the carb. When you shift from idle to gear, there's a big drop in RPM (from 1000 to 600)... This makes me think this big cam is trying to fight a stock torque converter!
So we have a car that feels like it makes 100HP, won't rev past 3500RPM, and is a total PIG. I'm scared it's way over cammed, stock converter, and highway gears. I don't have the specs on anything, so no way to confirm. If this is the case, wouldn't I still be able to rev the engine up to the 5500 redline?
Next steps are wide band to ensure the carb is tuned correctly, what else could be causing this massive loss of power? What should we work through next? It runs great at idle and low RPM cruise, but when you step on it nothing happens lol
Did you get a vacuum gauge on it? I'm not so sure that's a cam your hearing. I wonder if you have a legit misfire. Did you check your wires and plugs? Also make sure you don't have anything crossed up plug wire wise.
Did you get a vacuum gauge on it? I'm not so sure that's a cam your hearing. I wonder if you have a legit misfire. Did you check your wires and plugs? Also make sure you don't have anything crossed up plug wire wise.
We did double check the firing order, then switched plug wires out to make sure it wasn't a "4-7" swap cam: the firing order it correct.
Is the cam too big for a low compression engine? Did someone install a big cam with stock springs and are we getting valve float?
We did double check the firing order, then switched plug wires out to make sure it wasn't a "4-7" swap cam: the firing order it correct.
Is the cam too big for a low compression engine? Did someone install a big cam with stock springs and are we getting valve float?
The cam might be too big but it should still run out if everything else is right, it just would take forever to run out. If it was a legit 5.0 HO motor with the original heads (601s with 53 cc chamber) and was rebuilt roughly in stock specs the static compression should be around 9.5:1 on that motor. Which should run all but the largest cams ok.... Now if you have the other 305 heads (416s which has a 58 cc chamber) then the stock compression is closer to 8.5:1 which you would definitely have a problem with anything other than a RV cam. I would get some vacuum readings and compression tests before I went any further.
The cam might be too big but it should still run out if everything else is right, it just would take forever to run out. If it was a legit 5.0 HO motor with the original heads (601s with 53 cc chamber) and was rebuilt roughly in stock specs the static compression should be around 9.5:1 on that motor. Which should run all but the largest cams ok.... Now if you have the other 305 heads (416s which has a 58 cc chamber) then the stock compression is closer to 8.5:1 which you would definitely have a problem with anything other than a RV cam. I would get some vacuum readings and compression tests before I went any further.
Interesting you bring this up, because I did confirm it had 416 heads today. So this means, it's likely low compression. Looks like a compression test is next!
Interesting you bring this up, because I did confirm it had 416 heads today. So this means, it's likely low compression. Looks like a compression test is next!
another point of view, a motor with low compression, but long duration cam is perfect for a supercharger.
Maybe that was the intended goal for whoever was building it.
slap a wieland 144 on there! Lol
You need to take post # 8 very seriously before tearing things apart.
Later model C3s IGN & FUEL systems need a brain to tell them what to do at every RPM. Without that data available to feed those two, the engine will run like yours is doing now.
Might be a good time to go old school carb and distributor.
You need to take post # 8 very seriously before tearing things apart.
Later model C3s IGN & FUEL systems need a brain to tell them what to do at every RPM. Without that data available to feed those two, the engine will run like yours is doing now.
Might be a good time to go old school carb and distributor.
I understand it used to have computer controller ignition and carb, but now it just has an aftermarket HEI and a Holley carb. Is there anything else that could be interfering?
Engine won't rev and makes no power!
