J20 401 Rebuild

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Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

J20 401 Rebuild

Post by FSBOB »

Hello everyone,
I've got a 401 I'm looking to rebuild this fall. I'm currently running sniper efi, msd ready-to-run ignition, edelbrock performer intake and headman headers. It mostly works well together but I did end up adding a 1" spacer between my throttle body and intake after installing the performer because I had issues. Turns out my throttle body needed a good cleaning but there are a lot of efi threads that talk about having issues with the efi not liking dual plane intakes. I'm still playing with mine so I haven't figured out anything positive or negative yet but I have found that after installing the performer intake I had to advance the timing to 15 ish degrees at idle. It no longer backfires at partial to full throttle with this change. Any ideas why that might be?

Also, I drove to work one day with the hood off and could see smoke coming out of the breather every time I was at WOT hence the rebuild. What I'm wondering is for you guys who have rebuilt 401s before, where have you gotten your parts and things to look for?

I know the 401 was throttled back after 71. Did they do this by using dished pistons and lowering compression? I'd like to get more out of my engine without using a wild cam and losing vacuum at idle, so if you use higher compression pistons at what point do you think I'll have to run higher than 91 octane fuel?

This motor has been rebuilt by a previous owner and I'm not sure what was done before, but as of now my plan is to bore 30 over and install high compression pistons/rings, all new bearings, rv cam, double roller timing chain, oil pump rebuild, and although I'd like to install edelbrock performer heads I can't afford them and plan on rebuilding some stock heads I'm getting from a friend off a spare 360.

FYI, this is my daily driver for now but I am not worried about fuel economy, mostly reliability... and the ability to do burnouts on 33's... :D

Any and all suggestions/insights/help is greatly appreciated.
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:

Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

Re: J20 401 Rebuild

Post by FSBOB »

I've been digging into my backfiring issue some more and I may have done something boneheaded. When I reset my idle after replacing my intake manifold I didn't disconnect the vacuum advance. Would this potentially be my problem? Reading through setting timing in the technical service manual it says to set timing to 500rpm with the vacuum hose disconnected and then once it's connected to set the idle to 750 for manual transmissions.

I pulled the data log from the efi and every time I get on the throttle the AF/R jumps over 16 and when I let off it drops to 10. I'm thinking my timing may be way off because I also show my estimated volumetric efficiency at idle against AF/R and MAP is 60% or less.

I'm going to check this when I get home. I'd like to get more out of my truck but I'm definitely of the mindset if it ain't broke don't fix it. Any and all help as always is greatly appreciated.
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:

Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

Re: J20 401 Rebuild

Post by FSBOB »

Well, I set the timing while at 500 RPM and the vacuum advance disconnected when I got home yesterday and it seems to have fixed most of my issues. I'm not sure if I'm still getting blow by but I can definitely tell it's running much better. Still interested in any input you guys have on engine rebuilding but if the trucks running well I'm probably going to hold off until I can afford edelbrock cylinder heads along with the rest of the rebuild.
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:
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fulsizjeep
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Re: J20 401 Rebuild

Post by fulsizjeep »

Yeah, hold off on rebuild plans. On timing... not only disconnect the vacuum advance but plug the source. Checking everything for vacuum leaks is a good idea too.
Flint Boardman
88 GW, 401/727/208, 5" lift, D44s/4.10s/locked up, 35s
https://jubileejeeps.org/quadratrac

Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

Re: J20 401 Rebuild

Post by FSBOB »

Yep, did that and have replaced or plugged all vacuum lines. I did learn this weekend that MSD Ready To Run distributors are not compatible with EFI setups. I mean you cannot run timing through the efi computer if you are using a ready to run distributor. Caused myself a lot of headaches and an extra 10 hours of routing wires and troubleshooting to figure this out. All my fault but it's running good now. Anyone running msd distributors, what size springs/spacer are you using for your centrifugal advance?
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:
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derf
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Re: J20 401 Rebuild

Post by derf »

I locked out all mechanical advance on my MSD distributor when I went to EFI. I put an 8 pin GM ignition controller inside of a gutted factory ignition box in the factory location.

I reused the factory wires going to the distributor and to the coil. I hooked them up to the pins on the GM controller. I then ran the other wires to the EFI computer.

Everything seems to work fine as far as the computer controlled timing goes.

Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

Re: J20 401 Rebuild

Post by FSBOB »

I won't lie to you, I can't even begin to understand how that would work. It seems to be running well with the msd controlling things as long as it's timed right....
I do have an oil pump rebuild kit, new radiator and electric fans, power steering hoses and other small things coming to keep me busy. Since I don't have to worry about the engine losing power im trying to make it more drivable since it is my dd. Also going to try and rebuild the steering box and steering pump.
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:

Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

Re: J20 401 Rebuild

Post by FSBOB »

Well,
Turns out I'm going to have to tear the motor apart anyway. Found brass and silver in the oil doing the oil change this weekend. Not planning on doing anything wild, staying with stock compression/heads and everything. Any additional advice you guys can give me before I start pulling the 401 out and tearing it down this weekend will be welcome.
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:
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tgreese
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Re: J20 401 Rebuild

Post by tgreese »

The bore spacing of the 401 is really close, and there's not a lot of iron in the cylinder walls. Not like a typical gray iron block like the 360 or 304 that will go 30 then 60 routinely. Seems like some go oversize fine, and some don't. Sonic check if you're thinking about boring the cylinders, and then maybe don't. If the walls will be too thin, you can always sleeve, but that will be at least $100 per hole.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
FSBOB
Posts: 23
Joined: Fri Jun 23, 2017 6:30 pm
Location: Minnesota

Re: J20 401 Rebuild

Post by FSBOB »

Thank you for the advice. I'll mention that to the machinist when I drop it off.
1976 Jeep J20 401, T15, Dana 20, Holley Sniper EFI, Hedman Headers, MSD Ready-to-Run Ignition, 33" Cooper MTP's :fsj:
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