Author Topic: Tightening up when hot. (Read 2885 times)

Retro Rocket · « **on:** August 20, 2013, 03:41:21 PM »

Morning all, Just thought i would run this past you guys and see what the general consensus is.

I was talking to a guy at a bike show on Sunday, he owns a tidy F1 750/4 with a MTC 900 kit and running VM29 Mikuni smooth bores. He was telling me that he hates riding it i traffic because after it gets hot it won't start with the electric start, he blames the forged pistons swelling in the bores as they heat up, I am not so sure. I've had almost every kit possible in Honda's over the years including the 900cc kit and never had this problem at all, I am currently putting together a K model 750 and I am using this exact same kit, MTC900cc. What do you think...?

This is the bike in question...

Jerry Rxman Griffin aka MuthaF'er · « **Reply #1 on:** August 20, 2013, 04:01:57 PM »

I'd say it depends on how tight it was bored and if the rings were sized correctly. Looks lean too.

Nice bike!

Retro Rocket · « **Reply #2 on:** August 20, 2013, 04:04:08 PM »

Quote from: Jerry Rxman Griffin on August 20, 2013, 04:01:57 PM

I'd say it depends on how tight it was bored and if the rings were sized correctly. Looks lean too.

Nice bike!

I suggested a lean condition but its well set up carb wise, plenty of fuel and good color in the pipe, I didn't mention it but i suspected a too tight bore...

Jerry Rxman Griffin aka MuthaF'er · « **Reply #3 on:** August 20, 2013, 04:17:21 PM »

Pipes look too discolored to me. Jet it slightly rich and see what happens. Perhaps the 29's are not flowing enough air. I can see where the mixture could be "right" for those 29 carbs but the bike could still be lean. A set of 31's might help

Retro Rocket · « **Reply #4 on:** August 20, 2013, 04:26:12 PM »

Quote from: Jerry Rxman Griffin on August 20, 2013, 04:17:21 PM

Pipes look too discolored to me. Jet it slightly rich and see what happens. Perhaps the 29's are not flowing enough air. I can see where the mixture could be "right" for those 29 carbs but the bike could still be lean. A set of 31's might help

The VM29's were used as a hipo upgrade on Z900, GS1000, Z1000 and CB750 so i doubt there's much trouble with flow, they are wearing Lynx ramflo filters which are less restrictive than stock so that side of things should be ok, he doesn't have a big cam in it either, I'm still leaning towards too little clearance in the bore.... Thanks for the comments though.

bwaller · « **Reply #5 on:** August 20, 2013, 04:38:08 PM »

Is it slow to turn over or just won't start with the e-start? What about the lever? If it's tight it ain't right!

I know the big Kawas used small carbs stock and the 29's were the hot ticket but these Honda engines seem to need bigger carbs. RR, I run those carbs on my 572 and they're a very good combo except for a slight spot at 5500. It idles well too.

Regardless if it's tight when hot the problem is in the bore. My forged pistons showed signs of being tight at 0.0035". They're now 0.005", these grow like crazy in the heat.

Retro Rocket · « **Reply #6 on:** August 20, 2013, 04:56:55 PM »

Hi Brent, it struggles on the starter button but kicks over fine, I don't think its getting excessively hot. The VM29's I have just came of a 900 kitted 750 that went just fine, besides i'm selling this one not long after its finished, have better bikes to fund.....

johno · « **Reply #7 on:** August 20, 2013, 09:35:10 PM »

Re the hot start,
In terms of engine tune or electrical problem ?
Engine tune, if the mixtures are fine maybe a less aggressive spark advance especially since the fuels are getting pretty ordinary but from what has been discussed with easy to kick over etc
my money will be on the starter or solanoid voltage etc as especially with the starter any winding issues tend to show up after a hot soak, pretty common so I would be checking that first, poor windings and more cylinder pressure will always raise the bar for a marginal starter, also check the charge rate ?
cheers Johno

scottly · « **Reply #8 on:** August 20, 2013, 09:46:58 PM »

My bike got so hot I couldn't kick-start it due to 40 low-speed jets in the Webers and a 70+MPH blast in 90* temps for 100 miles. Those pipes are blued from heat due to running lean, IMHO. If the bike kick starts with ease, but won't start with with the electric starter, it may be a battery issue, or a starter issue, as Johno mentioned.

Retro Rocket · « **Reply #9 on:** August 20, 2013, 10:47:15 PM »

Thanks guys...

scunny · « **Reply #10 on:** August 21, 2013, 12:26:02 AM »

my bike (650) developed the hot start issue, pulled the slenoid and dressed the main contacts and problem solved. just saying

Sam Green Racing · « **Reply #11 on:** August 21, 2013, 01:19:47 AM »

Something to remember guys, a lean mixture WILL blue pipes, when the mixture has been corrected, the pipes DON'T go back to a nice chrome colour.

The blued pipes could have been from a previous lean condition.
Some good reasoning in there though.

Sam.

Hi Scun, long time no hear.

trueblue · « **Reply #12 on:** August 21, 2013, 01:36:55 AM »

I would suspect the bores being too tight. But before I went and did any surgery I would check and double check the starting system. Either could cause a hot start issue but considering the fact that it kicks over easy I would check the electrics first as Johno said

754 · « **Reply #13 on:** August 21, 2013, 01:48:12 AM »

An overly lightened alternator may cause problems, if starter iis used often.

PeWe · « **Reply #14 on:** August 21, 2013, 03:37:12 AM »

My bike had the same problem.
I had to use the kick starter when hot. Had to use kick starter and throttle rather much. Engine always started at first kick with a really load sound from the rather open 4-1. After a refill on a gas station on a touring one hot Italian chick got ice cream in her face due to the sonic chock :-)

I got used to it and do not recall if it was worse with my first forged RC 836 than later cast Action Fours 836.
I have a 3:rd set in now. I checked the ring gap extra this time when I read about too small gap can be a factor too.
The bores must be wider now after hone with Flexhone brush.

Tintop · « **Reply #15 on:** August 21, 2013, 10:42:13 AM »

Given it kicks over easily, and the condition happens after riding in traffic, I'd be looking at the charging, then starter systems.

voxonda · « **Reply #16 on:** August 21, 2013, 01:43:00 PM »

Many moons ago had a same prob, boring was ok given the middle two cylinder an extra .004. What I discovered back then was that the startermotor was 'binding'(??) I dismantled it, reemed the bearing surface and sanded and polished the axle. Never had a problem since.

Retro Rocket · « **Reply #17 on:** August 21, 2013, 02:20:30 PM »

Thanks for all your help guys, i'll pass this on and see how it goes....

Tim2005 · « **Reply #18 on:** August 21, 2013, 03:14:59 PM »

As above, I strongly suspect his battery isn't 100% and a new battery with high CCA rating, like a Motobatt, would solve this.

However, I did do some experiments relating to this tightening when hot issue a couple of years back... using the approach of comparing the measurements from using a torque wrench on the end of the crank to turn the motor over hot and cold, on stock 400 and 750 motors and on a 460 too. I had expected a cold motor would be tighter, due to thick oil etc, but the hot ones required somewhere around 25 to 30% more torque to turn them over. A decent battery should manage this aok of course, but I thought it interesting nonetheless.

PeWe · « **Reply #19 on:** October 06, 2013, 12:33:56 PM »

Maybe battery, grounding as I have read on this forum and this thread. I saw when taking my bike apart that the ground (-) wire from battery was not perfectly grounded into frame and engine. Better now as it should be. The starter feels much more powerful now.

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Author Topic: Tightening up when hot. (Read 2885 times)