When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
So I just put a new crate motor in my 81. It is a full roller setup with about 425HP. I also had my TH350 tranny rebuilt. Since installing everything, during the break in drives I have the following bugs.
1. brakes feel like they are not power brakes
2. Tranny will not kick down into passing gear
3. Cannot feel or hear the vacuum secondaries open on the Holly 650
Can these bugs all be related to the motor not producing enough vacuum due to the big cam that came in the crate motor? I have posted a Pic of the cam specs.
Your first order of business is to go to Harbor Freight and buy a vacuum gauge to verify your theory.
On #1, yes that could be from low vac. Brake booster may work on down to 11 inches of vac. You may be lower than that.
On #2, I think some linkage was left out after the swap.
#3. No. Secondaries open at full throttle = zero vacuum.
You have a lot of cam there. I suspect you may end-up with a 12V vacuum pump incorporated to run everything.
Your first order of business is to go to Harbor Freight and buy a vacuum gauge to verify your theory.
On #1, yes that could be from low vac. Brake booster may work on down to 11 inches of vac. You may be lower than that.
On #2, I think some linkage was left out after the swap.
#3. No. Secondaries open at full throttle = zero vacuum.
You have a lot of cam there. I suspect you may end-up with a 12V vacuum pump incorporated to run everything.
Thanks for the input-
I am not that knowledgeable when it comes to carburetors but I was told that this carb had vacuum secondaries and the vacuum module for the secondaries was (see pic)
Does your car still have the vacuum canister behind the master cylinder? That also needs check valves to hold the vacuum. These canisters often have pinholes so they leak but that can be soldered or brazed.
p.s. I don't know if that system was gone by 81 or not.
Does your car still have the vacuum canister behind the master cylinder? That also needs check valves to hold the vacuum. These canisters often have pinholes so they leak but that can be soldered or brazed.
p.s. I don't know if that system was gone by 81 or not.
Thanks,
I will check that also, but the power brakes worked great before the engine swap. After the swap they still work but they are hard, like no power assist. I only see the brake booster behind the master cylinder
On vacuum secondary carbs, the vacuum more or less holds the secondaries closed until needed.
Without vacuum, which is half-throttle to full throttle, there is little to no vacuum.
But here's the thing. I see a secondary accelerator pump. That's a double pumper.
Thanks,
I only see the brake booster behind the master cylinder
The canister would be under the fender and alongside the master cylinder if it was there. Very hard to get to in any case. It's roughly cubic in shape, maybe 8" x 8" x 8"
That's a big cam.
I know a smaller duration cam than yours only made 10" vac at 900 rpm idle.
So I suspect your vac is much below that.
That would cause your PB issue.
Time for a smaller cam, manual brakes, or a vacuum pump.
Thanks for the input-
I am not that knowledgeable when it comes to carburetors but I was told that this carb had vacuum secondaries and the vacuum module for the secondaries was (see pic)
That module with the blue line is for factory cruise control I believe.
With that power band starting way up at 3200 I assume you put a high stall converter in the trans right? otherwise I would think it wouldnt be much fun to drive around on the street below the cams powerband. But with that Cam, driving on the street is not the primary goal right?
Last edited by augiedoggy; Nov 6, 2024 at 08:16 AM.
With that power band starting way up at 3200 I assume you put a high stall converter in the trans right? otherwise I would think it wouldnt be much fun to drive around on the street below the cams powerband. But with that Cam, driving on the street is not the primary goal right?
That's my problem, my cam was supposed to be it the 1500-4500 range
I cannot return it now so I am just trying to make it work. I think I need a smaller cam and get rid of the double pumper. I do not want to be driving around with a 3500 stall converter, it has a 2800 in it now
If you want a 1500-4500 rpm power range, and cam, it is time to bite the bullet and change the cam.
That cam is a monster (comparatively) and you will never be happy with it until you swap it out.
There is no "trying to make it work" here. That's about 30* bigger duration than your desires.
To heck with returning it, just buy a new one. They are only a couple hundred $.
Or talk with your crate motor supplier and at least have them ship you the right part.
248*/252* duration is huge. You want more like 224* for that powerband.
230* tops. Once you go over 230* duration, cams enter the "hard to live with" zone. And yours went way over.
OTOH I see no issue with the DP carb. You should not be able to "feel" it open, it should be smooth.
You can add a vacuum booster that will help with the brakes. The trans might need a kickdown cable adjustment. The carb probably needs adjustments for the low vacuum with that big cam. Try adjusting and tuning it before swapping cams.
So I just put a new crate motor in my 81. It is a full roller setup with about 425HP. I also had my TH350 tranny rebuilt. Since installing everything, during the break in drives I have the following bugs.
1. brakes feel like they are not power brakes
2. Tranny will not kick down into passing gear
3. Cannot feel or hear the vacuum secondaries open on the Holly 650
Can these bugs all be related to the motor not producing enough vacuum due to the big cam that came in the crate motor? I have posted a Pic of the cam specs.
Since everyone else is spending your money to solve these problems, I'll suggest:
1. Convert to manual brakes
2. Convert to a manual transmission!
3. Convert to EFI
Or you could change the cam. Did the engine builder not meet your spec/quote/contract? That's on them to change it.
The canister would be under the fender and alongside the master cylinder if it was there. Very hard to get to in any case. It's roughly cubic in shape, maybe 8" x 8" x 8"
I believe in '81 the vacuum reservoir is behind the front bumper cover, long tube looking thing.
Lose all of the vacuum stuff and you'll be better off.
Even if he butchers the car and loses the power brakes headlights vac advance and the rest.... He still has to deal with a cam that will be a dog till the engine hits 3200 rpm because its the wrong fit for the car. It makes way more sense to just address the problem rather than trying to change the car to work with the cam and still be no fun to drive.
New Crate Motor
