Bonneville America Forums Forums Technical Forums General Tech Power loss below 4000 rpm

Hey guys. I know I haven't been on the boards much lately, I still have very limited internet access since my summer trip. Here's the problem. Lately I have had a noticeable power loss below 4000 rpm. In addition, at idle, there is a slight burbleing coming from the right pipe. I'm also getting the occational cannon shot of a backfire on decel, again sounding like it's from the right side. Above 4000 the bike runs beautifully and it doesn't seem to run rough at the lower speeds at all, just a loss of power. I did a tune up recently, adjusted the valves, cleaned the plugs, cleaned the air filter, etc. Any ideas? I have had almost 60,000 miles of trouble free riding... HELP!

Have you balanced the carbs?

Check the vacuum caps on the carbs for tiny cracks. Or better yet, replace them all with crossover hoses.

Good call Pat. I am thinking vacuum leak also. Another thing to check for leaking is the rubber connectors between the carbs and manifolds

Why put a cross over hose?? I've herd a few people talk about doing this and was wondering what it does??

The carbs have been balanced. The vacuum caps all look ok, but I guess it couldn't hurt to simply replace them. The carb boot all look good as well. I too am curious about the usefllness of the crossover hose.

Thanks for the input guys!

It equalises the intake vacume. Used to do it on the old bonnies. Souldn't need it if the carbs are balanced properly.

When I balanced the old carbs I used to back the idle screws all the way off stick a finger in one carb to feel the slide and look in the other and twist the throttle when they both lifted at the same time they were done.

it will also last a lot longer than the squirrel condom's I have had a cross over tube for the time I would have replaced the caps 3 times. I would use nothing else now

I am having the same issue. I changed my pipes out to BUBs, rejetted the carbs, changed the needles, and shim, with the dynojet kit. What does cross over hose look like, what size would you use, and where would you connect? Does anyone have a PIC?


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04-America, Dynojet carb kit, AI removed, BUBs, UNI, top snorkel removed, drilled air box

It connects to the little vacuum ports on the carbs. Connect them left to right, linking the carbs. There are 4 ports, so two tubes. I'll see if I have a pic... You can use vacuum hose from an auto parts store, 1/4" I think.

So, if read I this correctly, when you do the 2 cross-over tubes linking the carbs, this would eliminate the need to occasionaly balance the carbs...correct? They would always be synchonized because the adjustment screw would be by-passed by the tubes.

no. The carbs still need to be balanced. If not you will draw more fuel from 1 than the other. However if they are slightly out that hose will make it so you won't notice. The butterflys still have to open at the right time and the same amount to regulate fuel so no it doesn't bypass the screw.

OK, cool...I'm learning a lot here
Thanks

Actually on the balancing note. When you balance be sure none of the cross over hoses are in place or you will get an incorrect reading.

Thanks again

Vacuum leak is the first suspect. After that, at around 3500 - 4000 is where the slide opens enough that you are running on the main jet. Below that, you are running on the idle and transition jets which are fed by the pilot jets. If it idles well with the idle screws around 2 1/2 to 3 1/2 turns out, the idle jets are probably OK. Take a look at the transition jets, 2 (maybe 3) holes in the bottom of the carb throat just before the throttle plates. Also make sure the pilot jets and related passages are clear and properly seated.

I think it's karma. You have a beautiful set of green/cream tin sitting in the shed needing a good home. If you were to sell it to another member (ahem), I'm sure the bike would run much better. Do you Paypal?

Regards from one Stubborn Polock

LOL... Nice try! I actually put the Green and Cream paint kit back on the bike just last week! Ahhhh... looks like it should again. Looks good parked next to the Thruxton too!

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I actually put the Green and Cream paint kit back on the bike just last week!

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Tired of the Tangerine/White? If so, why? I've been concidering that color combo for some time now.

Leaky vacuum nipples will cause some probs. My bike exhibited some minor rough idling last week specially in the afternoon when temps got high. It would also on three or four occasions just quit. It would start right back up... but.

Yesterday I did some standard checking battery terminals for tightness and I put two new rubber squirrel condoms on. The old ones upon close examination had some cracks and when I pulled them off they came off in chunks. Those little rubbers should probly be R & R at about four month intervals as opposed to eight or nine.

You took off the Tangerine paint? I know a gal named mert who would kill for a set!!

The Tangerine and Pearl Grey was a temporary paint job from the start. I didn't want to risk the Green/Cream paint during the summer trip so I did the quickie 1959 style paint before I left. I may use the repainted tins again as is at some point (next cross country) or I may paint them in 1964 Bonneville colors first. I Love options!

Problem solved. Seems the local gas stations have switched over to their summer blend... and it's worse than before. I put the bike on the gas analyzer and it was running so lean it was a joke. A little fudging with the idle mixture and bigger pilots and I seem to be bake to normal. God I hate California... and it's gasoline!