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On my Suzuki '81 GN400 4 stroke I cleaned the carb -new mikuni VM less than 600 mi on it, (used compressor), set the valves, adjusted the timing chain, checked the gas flow and it all looks good. The bike kicks over and starts right up, idles great. I go up the road with it- it doesn't like to get above 4000 rpm - no matter what gear (top speed 40-45 mi!)- starts to get louder and deeper and I need more throttle to get a response. On the ground it will hit 4500, 5500 rpm no problem. The bike doesn't have the zip it had. It reminds me of an old truck I had when the electronic ignition was dying. Is it the CDI? The plug is black with dry carbon. I put a new coil on it when I got the carb for it. I checked the wiring earlier- it all looks good. This is a 6 v bike. What does a voltage regulator do? The electrics are 25 yr old and it sat for at least 20 yr of it in a shed. 8,900 mi on the bike.
-appreciate any ideas.

It could be the carb. The mainjet and needle settings may be set too rich. Does your carb needle have a clip and various slots to change the hieght when assembled? Is it all back together the same way it was before?

Also, a restriction in the air intake can allow a bike to run fine at lower rpms, but choke it off at higher ones. May want to check your filter and airbox.

And now I'm out of guesses.....

Good guesses Buzz

A typical bike Voltage regulator is a thyristor takes all the excess voltage from your alternator, sends about 14v to the battery, and dumps the extra voltage as heat. It shouldn't have anything to do with your problem.

Pay attention to the throttle Position when this happens. If it happens at specific RPM it could be electrical, but if it happens at specific throttle position, it's likely the carb.

have you checked timing with a timing light to see if it's advancing?
-ding ding-

-thanks on the voltage regulator- good to know


-that's good to know on the rpm vs throttle whether it's car or electical

it has a tough time geting to 4000 and 4500 rpm pretty predicatble - I'm leaning to CDI

I'll pull off the air filter- a KN add on and see what it does, but it looks pretty clean

I appreciate all of the ideas- keeps my head going and thinking up new angles

how do you use a timing light on the bike-- the magneto cover hides the timing TDC mark - is there something else I would flash it at?-- I don't see any way to change the timing- I think the cdi does it- the timing feels mentally challenged to me -bogging down. Is there a static way to time it?

thanks again

you state that it is a 6 volt bike--1981 model???????????????

If the timing is off, the CDI is bad. No adjustment (I dont think), just replacement.

how about just removing the cover and flashing a light on the flywheel?
I have seen this exact same problem with a honda rebel, turned out to
be that the ignition system was not advancing. either way, if you can
replace the cdi box with a known good one, just try that.

It's been a bit- but the CDI I ordered from Bikebandit.com finally came- original suzuki packaging---'81 GN400 4stroke- single cylinder----I stuck it in tonight after work - problem fixed -- no more dead zone rpm above 4000 --no more top speed of 40-45 (in fith!) no more getting run down on the road!

Runs even better than it did when I first got it going last spring! smoother -- cracks in the back of the old cdi unit - I think it has been a slow process- moisture? cold in the air made it worse?

Expensive part- I have about 1200 in this bike now-- new parts tires,chain,electrics,carb, paint, helmet, insurance etc..but it is still cheaper than a new bike--though my dream of a new one gets farther away with each repair! -no reason to! (two years of riding to break even over the money I save on leaving the truck home. -cold winters here!)

Also have a Suzuki GN 400X with the same lack of power at higher RPM,s. I also replaced the CV carburtor with a Minkuni VM carburator. This wasn't the problem , however I don't regret the change as the diaphram in the CV was showing wear and it was a $110 part, and other parts for the older carb were not available.
I had supected the advance circuits were bad in the CDI for some time however as you stated because of the construction of this engine one cannot strob light the flywheel timing mark without removing the magneto stator. Bike Bandit price for this CDI is now up to $232 so I wanted to be certain that this was the problem before ordering. The only possibilities left as to the lack of RPM power that were left was a lack of ignition advance from the CDI or incorrect valve timing ,caused by the timing chain being off a tooth or more. To check the valve timing I had to remove the cylinder head cover to to check the alingment of the sprocket mark at TDC. The valve timing was OK.

A this point I figured out a way to check to see if the CDI was advancing using a strob light. There is a rubber plug seal at the magneto side of the engine which seals off the left end of the camshaft where the old contact point assemble was once installed on the engine on 1979 and earlier 370cc units pre CDI. I remove this plug and replace the cylinder head cover. Put a chalk mark on the end of the camshaft and aimed the strob light at it . Reving the engine reveal that there was not any ignition advance at all. One has to to this rather quickly as some engine oil will be lost from the camshaft end when making this test.

When the timing advance circuit is bad in the CDI the ignition is still very good as to starting and low RPM use making it a hard problem to locate. The Suzuki dealer said he haas never had to replace the CDI unit, but he did not have the Suzuki test equitment neccessay to check the CDI with. I feel many of preformance problems are being blamed on carburation etc. when really it is retarted ignition timing. I did use an ohm meter to test the CDI as explained in the service manuel and it revealed the resistance to be much higher in parts of the CDI than specfied in the manuel , the conditions were worse if the CDI unit was chilled down the the freezer and better if warmed up withe the hair dryer. The CDI is on order.