Any experts on small capacity Suzuki 4's?

[Blackbird]

Late last year, a mate bought a lovely 1983 Suzuki GSX 400F from Taranaki. It was in nice order to start with, having done only 33000 km but a few hundred bucks on paint and other odds and ends has turned it into a genuine stunner - see pic.

It runs sweetly below 5000 rpm but as the revs and load builds, it goes "wooden" as opposed to misfiring and doesn't seem to want to go much beyond 120 km/hr. My mate is very experienced around motors but he's currently stumped. He's put in new plugs, compression is fine, as is the ignition timing using a strobe. He's also swapped coils and HT leads. He has not checked the CDI unit. Air filter is clean. Fuel feed is fine and he's dismantled and rebuilt the carbs with no change in performance. He has yet to look at cam timing but as a cause, it's improbable. There might be something obvious that has been missed, or something particular to the Suzuki 400 motor.

Anyone like to venture any ideas or further avenues of investigation?

Cheers,

Geoff

[xwhatsit]

Valves? I read an article recently about an engine which was doing similar things -- turned out that the valve springs were stuffed, or not working properly or something, and the tappets needed a fair bit of adjustment. It would make sense in terms of the power being good below a certain RPM, but as the revs increase maybe valve float or something? Or if the tappets are out or cam is really worn, then the valves aren't opening enough to get enough fuel in above a certain RPM? I don't know, I'm no mechanic.

[Ixion]

Is that the standard exhaust?

[Blackbird]

Compression is fine, as are clearances. Hmm....the exhaust. According to my mate, it's stock under the black paint. I wonder if it is because it looks a bit "basic" to me but I'm probably wrong. If it's strangled, that could be a strong contender. Definitely worth some experimentation methinks. Thanks for the responses so far - much appreciated.

[JMemonic]

Hmm....the exhaust. According to my mate, it's stock under the black paint. I wonder if it is because it looks a bit "basic" to me but I'm probably wrong. If it's strangled, that could be a strong contender. Definitely worth some experimentation methinks.

It looks more to me like an after market 4 into 1, a mate had one of those brand new and it had a 2 into 2 system, he fitted a similar product and it was so much louder from the original exhaust until it clogged up and started rusting internally, was very poor quality, he ended up building a new baffle assembly for it.

[Sensei]

Looks like a old Sager system from the 80's . As above would check that it hasn't blocked itself up , maybe cut it open in front of the buffle area & try it with an open pipe to see if it will pull full rev's if so then it has become blocked or is to restrictive .

[Motu]

Sort that exhaust,then look at the carbs - a ruptured diaphram will run fine at low speeds,but limit revs.How is he checking timing? What is the ign timing at 5,000rpm and beyond? Set the timing for max rpm and work back....checking at idle is fine if you want to idle all day.How ''fine'' are these compressions? Cam timing out is not common on a chain motor,but you never know who has been there before - an advanced inlet cam give very high compressions.

[FROSTY:Me]

my money is on the exhaust--I bet its been blocked up at the end to get it quiet enough to pass a WOF --but itll be too blocked for high rpm

[Blackbird]

Thanks everyone - that's great. I've managed to find a picture on the web of an '83 model and the muffler is a sight larger than the one that is on it. That's the first (and easiest) step then. I'll let him know and give some more feedback in a few days.

Thanks so much for your interest and help.

[roadracingoldfart]

The exhaust system fitted is without doubt a generation 1 slash cut CYCLEWORKS 4 into 1 and it does has a removable baffle so it can be repacked. Its a bitch to remove but it does come out eventually.
I had one fitted to my old race bike (1976 cb400/4) and it was absolute crap and stopped the bike revving out but it did shut it up for WOFs etc when i used it on the road.
I welded a turboflow accessory car muffler on and jetted it properly and it was worth another 2 secs at Manfield lol.
Id advise to ditch the pipe.
PS; they were all either black or chrome plated from new.
Cheers Paul.