Author Topic: Dolmar 7900 Porting Info  (Read 3796 times)

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Offline KilliansRedLeo

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Re: Dolmar 7900 Porting Info
« Reply #160 on: July 07, 2014, 09:47:27 am »
Adam, after looking very closely and doing some quick measurements, mine appears to have the same measurement from spigot to squish on both the IP & EP sides perhaps within +/- 0.0005. It also appears that the combustion chamber is a little off center shifted toward the IP centered on the plug and then tapers gradually toward the EP. Looks to be done intentionally because as near as I can tell it is equal measurements L to R in the bore. I'll try to get a better picture for you.

OBTW all the remaining NOS Solo 665s are enroute, should all be here by Monday next.
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Offline aclarke

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Re: Dolmar 7900 Porting Info
« Reply #161 on: July 07, 2014, 12:28:36 pm »
Thanks for measuring your Tim.  After studying a few old stihl work Saw pistons last night I think the carbon pattern on the top of the piston indicates a pretty thorough burn that cools down towards the exhaust side. This is a good thing that the offset chamber contributes to. 

Offline KilliansRedLeo

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Re: Dolmar 7900 Porting Info
« Reply #162 on: July 07, 2014, 12:43:21 pm »
Yeah, now that you mention it, same thing was done on the Maico 250cc MX bike jugs or manbe it was the CZ?
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Offline srcarr52

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Re: Dolmar 7900 Porting Info
« Reply #163 on: July 07, 2014, 02:40:41 pm »
Thanks for measuring your Tim.  After studying a few old stihl work Saw pistons last night I think the carbon pattern on the top of the piston indicates a pretty thorough burn that cools down towards the exhaust side. This is a good thing that the offset chamber contributes to. 

Generally the burnt locations are the hotter areas.  You'll have clean spots where the fresh a/f charge from the transfers cools and cleans the piston.

Offline aclarke

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Re: Dolmar 7900 Porting Info
« Reply #164 on: July 07, 2014, 03:58:15 pm »
that makes sense, thanks for clarifying that   looked at two well used 046 pistons and a 660 and they are completely black minus a very small area where the leading edge of the transfer opens. Dolmar seems more limited to the center and exhaust side, perhaps the higher CC pressure through the tiny transfers helps?  thanks, Adam

Offline KilliansRedLeo

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Re: Dolmar 7900 Porting Info
« Reply #165 on: July 07, 2014, 04:09:17 pm »
You, know Adam, I spoke to Shep=Farmboy this afternoon and he mentioned the same thing, smaller CC volume plus tiny transfers=higher velocity of the incoming charge. When Randy gets back next week perhaps he will provide his thoughts. I'd really like to come up with some sort of plan to port a 665 but haven't collected enough info yet. Also got news from the Solo distributor this AM, he thinks he might have a 681 top end kit floating around. He said it did not have the decomp valve, don't know if that meant no valve or no provision for one. We will see supposed to call back later today or tomorrow.
Tim
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Offline srcarr52

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Re: Dolmar 7900 Porting Info
« Reply #166 on: July 07, 2014, 04:45:10 pm »
that makes sense, thanks for clarifying that   looked at two well used 046 pistons and a 660 and they are completely black minus a very small area where the leading edge of the transfer opens. Dolmar seems more limited to the center and exhaust side, perhaps the higher CC pressure through the tiny transfers helps?  thanks, Adam

Completely black is bad... you want a piston with some wash similar to what you posed except more even from side to side.  It looks like the transfers are coming in at different angles and that is why it's not symmetrical. 

Overall your piston wash looks good, ignition timing is good, piston is the right temperature and your oil mix looks good because you have slightly more carbon buildup right under the spark plug and it lessens as you get towards the outsides of the piston in each direction because that is where the heat can be removed by the cylinder wall.  If you had a lot of carbon build up towards the ex side that would suggest late timing or too much oil and the gasses are still burning as they are going out the door. 

It being clean from the transfers all the way to the edge on the intake side suggest a good operating temperature, and that the fresh a/f charge is not leaving an ex pocket on top of the piston on the intake side which can happen with too steep of inlet angle in the transfers.

So like I said everything looks great except for the PTO side, I would like to see the line from the transfer divider like you see on the other side.  Something is messing with the flow from the transfer on the mag side closest to the exhaust port and causing it not to hit the top of the piston there.  It could be inlet angle, plating flash, or later timing then the other ports.

Offline jmester

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Re: Dolmar 7900 Porting Info
« Reply #167 on: July 07, 2014, 05:47:23 pm »
+1 to srcarr52.
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Offline KilliansRedLeo

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Re: Dolmar 7900 Porting Info
« Reply #168 on: July 07, 2014, 06:23:00 pm »
+2
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

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Offline farmboy

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Re: Dolmar 7900 Porting Info
« Reply #169 on: July 07, 2014, 10:21:49 pm »
Just for info the metal insert for the 7900 intake boot and the Husky 372 is a swap.  When I converted my 6401 to a 7901 I found the insert missing when I tore it down used the 372. 
Shep

 

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