backfiring

[Grudev]

Hi,
I got this kz1000 which i just finished a rebuild. it used to backfire a lot when it was still running, a long time back. i did the valve clearances and set them to the max spec.
it still backfires, not as much though and with much less intensity.
i use premium fuel.
i know my straight pipes with no mufflers are not helping matters a lot. cleaned the pistons and the cylinders too and ofcourse a carb rebuild.
and how do u post pics here?

[RichLockyer]

How tight were the valves?
If the clearances were too tight for too long, they could have been burned.

[zoro]

What year? Points or Pick-ups? Sounds like ignition problems. And/or as Rich indicated, if those valves were tight and hanging open for any length of time, carbon builds up on the valve edges and they wont seal properly once clearances are reset. Give the bike a few red line runs through the gears to burn and blow off any carbon that built up and recheck clearances.

[Grudev]

hey its a 81 kz1000 k ltd, points, which i replaced btw as one point had failed. i had leaned out the main jet needle as it was running a bit rich earlier, but now im back to 3/5 groove, since i thought lean adds to backfires. timing could be off, i dunno. what else could cause backfires. i tried to get some of the carbon of the valves too with carb cleaner, which worked pretty well actually.
i only got 2 more days to fix the bike the best i can, after which i leave the country for a long time.

[Grudev]

hey richlockyer clearances were still measurable on the exhaust side but 0 on the inlet, but not too far below 0.

[ArthurGoVroom]

Make sure that your exhaust pipes aren't cracked and that the crush gaskets in the head are new and in good condition and the bolts are tight. An exhaust that sucks air into the primaries will cause backfiring especially on deceleration.

[Grudev]

gaskets are new and bolts tight, but they are just straight pipes

[northernman]

Originally Posted by Grudev

hey richlockyer clearances were still measurable on the exhaust side but 0 on the inlet, but not too far below 0.

How do they get below 0 ???

[northernman]

Could also be a vaccuum leak, running lean, or timing way off.

[RichLockyer]

Originally Posted by northernman

How do they get below 0 ???

Happens all the time... it's when you have to change the shim size by 0.25mm to get the clearance to 0.15mm. You just don't know how far below 0 it was until you get it adjusted

[Grudev]

when u say a vacuum leak where is it implied? i have the clean air system disabled.

[Polar_Bus]

When does the engine backfire, accelerating or decellerating?

[Grudev]

idling for sure. And i'll have to pay closer attention for acc or decc, the thing is so loud i wear ear plugs while riding and my ears still ring afterward despite that.

[Grudev]

what r the probabale causes for those 3 cases anyways?

[StarGate]

Well since you're not running factory setup, what mods are done to the bike besides the exhaust. Obviously you've done something to the carbs so they're not stock but what? Are you still running the factory airbox?

[northernman]

Originally Posted by RichLockyer

Happens all the time... it's when you have to change the shim size by 0.25mm to get the clearance to 0.15mm. You just don't know how far below 0 it was until you get it adjusted

I meant the feeler gauge.

My set does not even have a 0, forget a less than zero.

[northernman]

Originally Posted by Grudev

when u say a vacuum leak where is it implied? i have the clean air system disabled.

And is it perfectly sealed off ?
If its able to suck in air, that could do it.
I take the decel add extra air goodies off my bikes to, or often they pop so much it sound more like Orville redenbacker, than a motorbike.

[StarGate]

Originally Posted by northernman

My set does not even have a 0, forget a less than zero.

Stop being so cheap and go buy one of the good sets.

[northernman]

Originally Posted by StarGate

Stop being so cheap and go buy one of the good sets.

My Bad.............Sorry

[Grudev]

hey yeah the feeler gauge with 0 is real nifty, its dimensionless!
anyways i used silicone sealant to the reed valve covers, ie the metal plates and then connected the wto with a piece of rubber tubing.
the carbs are i think v28 mikunis with sliders instead of the stock with butterfly valves. i cant remember the jet size, but the jet needle is on the 3/5 groove (stock) and air-mix screws are about 1 1/2 turn out. the filter is a custom made airbox. a cuboid with about the same projection area as the carbs and about 2 inches thick, the thickness has a metal mesh all around and inside a regular sponge sheet being used for filtering particulates, so that air only flows in from the sides.i doubt im running lean.