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Discussion Starter · #1 ·
Hey Fellas,

I'm a long time lurker here on the site, and have already learned a great deal from you guys, but need some help.

I have a 1976 CB500T (20,000 original miles) that I bought for next to nothing last year from a guy in Luray, VA. The previous owner said the bike shut off after a short drive and wouldn't restart. He took it in to a local dealer and a leakdown test revealed the bike had bent valves, so he put it up for sale as is. After I bought it from him, I took it home and tore the engine down to see what I could find. Sure enough, I found that one of the internal rollers had snapped, which made the cam chain loose, skipping teeth, ultimately screwing up the timing and bending the valves.

I ordered 4 new valves and took the head into a machine shop to have a valve job done and new valves installed. I also went ahead and had the jugs hones and pistons measured to make sure they're in spec. Everything passed according to the clymer manual and we did an acetone leak test for the new valves which passed fine.

Now for the rebuild; I ordered a replacement roller, new gasket kit, new piston rings, new cam chain, and did my best to install each following the manual steps. Intake and Exhaust Cam marks match LT on the left cylinder non compression stroke. (I dont believe the Cam timing has to be set on compression stroke, and the off stroke is easier to set since the engine wants to roll over past LTDC on the compression stroke, feel free to correct me). I adjusted the valves to spec, and believe at this point the engine is ready to go.

Now instead of going ahead and putting the engine right back in the bike, I get the bright idea that I want to make this a Cafe/Brat style bike and make a ton of cosmetic mods, new seat hoop and seat, new Mikuni vm32 carbs from DCC. Welded on new tabs for a new tank (different CB model), new clipons, controls, gauges, electronic ignition by pamco with e-advancer, powder coat the frame, all kinds of fun things.

Now, recently I put the engine back in the bike and started reconnecting all the wiring, all my indicator lights for gauges are working fine (with the exception of neutral indicator light/switch, I'll come back to that). New Pamco ignition installed following (Welcome to the PAMCO CB450 Installation Guide).

New carbs installed, pod air filters, stock exhaust pipes are back on, fresh gas, and guess what? No start. :(

Check for spark by pulling the plugs, grounding them to the engine, both have nice bright blue spark. I put them back in and turn the engine over a few times, pull them again, one is bone dry, one is covered in gas. Adjust the mixture screws a bit for each to richen and lean them out respectively. Still no start. run through these types of carb adjustments until my battery dies, then I drink a ton of bourbon and weep like a child lol. Recharge the battery over the next 2 days via trickle charger.

Few days later, I start doubting myself on the rebuild of the engine and check compression on the left and right cylinders. Both are pretty even, but also pretty low at about 115psi. I'm not sure if this is too abnormal since I just put on new rings that havent seated, but maybe thats my problem. I try a wet compression test and put a bit of oil in the cylinder, retest, and get 115psi in the right cylinder and 125psi in the left.

After several troubleshooting days, of adjusting carbs, checking spark plugs again and again, rechecking valve clearances on the compression strokes for left and right, I am completely stuck for what could be the issue. The problem is, that i've essentially touched or changed every component on the bike and dont know where to begin to troubleshoot, other than to run through the basic checks on spark, air, fuel, compression, timing.

I mentioned the neutral switch indicator light earlier, for whatever reason this light isnt working on my gauges, and i think its just a bad switch. I've got the bike in neutral, the back wheel spins freely, but is it possible that this model 1976 has a failsafe that would prevent the bike from starting if that indicator isnt working? I dont think thats related since its in neutral and I've got spark.

Any other ideas or thoughts you guys have for additional troubleshooting steps would be great. Sorry for the book, just wanted to provide all details. Thanks for the help!!
 

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Those compression numbers look low to me, but that’s not a dealbreaker. Did you do it with the carbs off? Your numbers can be artificially low if there’s any restriction in airflow. Carbs off is best, WOT and choke wide open is 2nd best.

Does the starter try to turn it and it doesn’t fire, or is it completely unresponsive? I would think a neutral safety switch would disable the starter and not the points.

Honestly, having been down the PAMCO road before (and finding that my PAMCO unit was faulty), the first thing I would do is pull off the Mikunis and the PAMCO. Get it running with the OE Keihins and the points. Then swap them out one at a time (if you’re married to the mods and don’t just ride off immediately :D), checking for perfect function once you have each installed. Getting these suckers running after a complete rebuild is an exercise is eliminating possible variables. Aside from that, it also sounds like your carb floats are set to the wrong height, if you’re getting a lot of gas on one side and less on the other.
 

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First and foremost, get rid of that Clymer manual. It's prettty much only good as a door stop.
Go here and download the Honda Service Manual instead
http://www.hondatwins.net/forums/54...uments/42637-cb500t-honda-service-manual.html

here's the direct link https://www.dropbox.com/s/3o3879gwa7hji98/CB500t_Service_Manual_Complete.pdf?dl=0


1. Pods are notoriously bad on these bikes. If you have the stock air filters still then I would suggest rebuilding those with Uni foam instead. Otherwise you will be in for a frustrating time trying to tune your carbs to run properly.

2. The cam timing must be done on the compression stroke.
 

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Also as mentioned before, there are too many potential problem points on your bike. I would suggest getting it back to stock condition as much as possible and go from there.
Here's how I would do it if this was my bike.

1. Re-do your cam timing according to the Honda FSM.
2. Re-check and re-adjust valve clearance according to the Honda FSM.
3. Use the stock Keihin carbs if you still have them. Clean out all the jets, adjust air fuel mixture, and adjust the floats according to the FSM.
4. Use the stock air boxes if you still have them.
5. Use points until you know the bike is mechanically sound and can start and run properly. Only then would I consider switching to Pamco.
6. Adjust ignition timing statically and adjust points gap as needed.
7. Make sure the battery is good. A fully charged battery at rest must read 12.6V or higher, anything less then it's suspect and should be replaced.
8. Check all electrical connections and make sure they are clean and snug. Make sure all ground connections are secured. Replace any frayed or broken wires as needed.
9. Check coils, plug wires and plug caps. Make sure they are not cracked, broken or loose. Replace as needed.
10. Check for sparks.
11. Start the bike.
12. If it still will not start, try spraying a little starting fluid into the intakes of both carbs and retry. If it starts then you have a fuel delivery issue. Take off the carbs and clean all passages and jets again making sure there is no obstruction. Make sure the float valve is working properly and not sticking. Check the floats themselves to make sure there are no leaks on them.

13. Once you can get the bike started, I would adjust and synch the carbs so that the bike can idle at around 1100 rpm. The CB500T Keihin carbs are already equipped with the vacuum ports to make synching extremely easy with one of these
https://www.ebay.com/itm/AlphaMoto-...382315089234?_trksid=p2385738.m4383.l4275.c10

14. Check ignition timing again with a strobe gun and adjust as needed.
15. Ride the bike and see how it performs through the different RPM ranges.
 

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Discussion Starter · #5 · (Edited)
First, thanks for the quick reply's.

@spirograph - I just did the compression test with WOT. I can certainly clean up and rebuild the Keihins, i do have the stock air filter too somewhere, but i was hopeful that the DCC mikuni's which are prejetted for pods would bolt on and work. I think i have the Mikuni's at least close now, their mixture screws were not even which is why one was so much more lean than the other.

@76Twin - I will definitely check out the other manual, thanks for the guidance there. Sounds like i should try to simplify the problem by going back to stock where possible. One follow up question for you on a comment you made, as that could be my issue. Why does the cam timing have to be done on the LTDC for the compression stroke? Isn't the cam to crank ratio the same on compression as it is LTDC for the non compression stroke? If it does have to be on compression, how do you guys keep the motor from rotating over on that stroke? I cant seem to keep it still.

Thanks.
 

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I just realized I kinda confused you on the cam timing comment. What I really meant was the valve clearance This must be done while at the compression stroke on LT.
I kinda lump setting the valve/tappet adjustment as a part of the whole cam timing process.

As for keeping the motor still as you rotate it, I have a little trick I used which makes doing ignition timing extremely easy




Basically I used a piece of wood as a stopper of sorts. It puts enough resistance on the rotor to prevent it from turning past the LT mark.
 

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Discussion Starter · #7 ·
Fellas - Just wanted to send an update here. As advised i went back and tried to simplify the problem. I removed the electronic ignition and the mikunis (after rebuilding the keihin carbs), set the points, and bolted on the old carbs/airbox, fresh set of plugs, and sure enough the damn thing started. Running a little rough and the carbs are definitely out of sync, but she lives and most importantly, the engine rebuild is sound.

I'll run through the new manual to sync everything up. But i'm pumped she's working. Many thanks for the tips above.
 

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Once you synch the carbs it will feel like a completely different bike for sure :)
 
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