Engine bogs down at RPMs above 3500

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Hey all. First post on this forum. Hopefully it makes sense.

Bike Info: Bike is a 1975. I rebuilt the entire engine myself. Running stock alternator with a Boyer electronic ignition. Kick start only with a lithium ion battery. Rebuilt the carbs, new fuel lines with inline filters, and rebuilt both petcocks. Am still breaking in the engine. Currently have just over 100 miles on it. Probably missing some info so just ask if more is needed.

Problem: Bike has run perfectly for 100 miles and then one day I was riding around town when I suddenly couldn't accelerate past 3500 rpm without the bike coughing and bogging down like it was low on fuel. Thinking this was the problem I rode to a gas station and filled up. No change. So I rode home keeping it under 3500 rpm. The next day I started it up and revved it past 3500 rpm and it revved fine and steady. No coughing or bogging down. So I took it out again only to have the issue show up again.

My thinking: Seams to be an issue only when the bike is in gear. The issue seams to affect both cylinders at the same time. I am running Uni air pods so air flow should be fine. I have made sure fuel is getting to the carbs. Both carbs have been cleaned and rebuilt and I find it hard to believe both carbs would get clogged at the same time. Could it be an ignition problem? It idles fine/runs at low rpms fine so ignition is working at least some what correctly.

I am pretty stumped as to what the solution could be. Wanted to see what anyone here had to suggest before I go tearing into the bike.
 
fuel cap venting?
try it without the inline filters. I'm smelling a fuel restriction issue. only shows up at higher flow rates.
75 has a shared choke with a cross over tube. check all of that, though an issue there tends to act up more at lower RPM.
 
I think Raymond might have had a similar issue with his Boyer. Seems he was running resistor plugs and spark plug caps and that was too much resistance. It made the Boyer act up.
 
Seams to be an issue only when the bike is in gear.
If you're saying that it revs OK in neutral but misses under load, yeah... sounds like ignition. 5twins suggestion above is a good possibility..... too high resistance in the secondary.
 
Seems he was running resistor plugs and spark plug caps and that was too much resistance.
Thanks for the quick responses! 5twins, how would I check and see if I am running resistor plugs and plug caps?
 
These are the plugs and plug caps I am using

Screen Shot 2021-08-07 at 6.34.40 PM.png
 
Yes, you've got too much resistance. The 05 in LB05EP means 5KΩ's.
The R in PBR7EIX means "resistor" ...also about 5KΩ's. You should run one or the other, but not both.
Look for either a BP7*** plug or a LZFH plug cap. Both are non resistor types.
 
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Since the resistor plugs are still readily available, maybe go with the non resistor caps?
I recently picked those up on Ebay.
EDIT: Just noticed he's now charging a crazy amount for shipping. It was only 3 or 4 bucks when I bought 'em. Maybe look for 'em somewhere else.


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@Jim Thank you so much for the help. I will go order those now!!!
Just checked that Ebay link and now it says free shipping. Better get 'em before he changes his mind... again. :cautious:
 
I had the same problem with my stock '75 a couple years ago. After many, many hours of diagnosis, I found that the multi-wire junction of the positive wire in the headlamp bucket leading to the coils had clean, but somewhat loose connections. I replaced the junction connector with a vintage-equivalent item and cured the issue.
 
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