Carb swap on 90 HP inline 6 Merc

[rideordie]

will the carbs and intake off a 150 mercury twister fit a 90 mercury inline 6?

[mariah2]

will the carbs and intake off a 150 mercury twister fit a 90 mercury inline 6?

The Mercury Twister that had the 1500 designation on the cowl was the Twister 2. It had 6 Tillotsen carbs. Technically speaking the intake which is the entire front half of the block would fit with enough work but it would be a miserable mess. The Twister 2 has main bearings on every crankshaft main journal instead of reed blocks on every other one like your 90. Your 90 goes top bearing, reed block, main bearing, reed block, main bearing, reed block, bottom bearing. The 90 has only those 3 reed blocks but the Twister 2 has the reed blocks in front of the crankshaft instead of on it and so it has 6 reed blocks. The crank would have to have bushings made where the 3 reed blocks from the 90 are. The front half and carbs are expensive and hard to find. If you went to the trouble you would have a mismatched mess because of the porting in the 90 but that is another issue. You would be better to sell the 90 powerhead and find a mid 1970s 150 inline powerhead or even an 1980s one if you prefer the ADI ignition. Modifying inline Mercury powerheads seldom produces the results intended and might result in an ill running engine with poor power unless done by an expert. Rumor has it that DaveS here has some experience and positive results from inline 6 building. It is best left to some one who has done it.

If you are thinking of the earlier 3 carb Twister the carbs would bolt to your 90 front case but would produce no gains. Bolt ons for a inline 90 will prove to be a lesson in frustration. If you can not learn to love it like it is set it free and find abetter match for your use.

[rideordie]

had a guy that said he culd do it but i wanted a second opion. ima just leave the motor alone and bore the exhaust and run a different prop. it has a 18pitch stainless on it have any idea about wat size prop will give a lil more speed.

[daveswaves]

had a guy that said he culd do it but i wanted a second opion. ima just leave the motor alone and bore the exhaust and run a different prop. it has a 18pitch stainless on it have any idea about wat size prop will give a lil more speed.

You can,t just bore the exhaust. If you have a full front half with all 6 tillotson carbs in good shape sell it on ebay and use the cash to upgrade to a late model 115 inline, you will notice the difference. The 90 is a great engine, you are just not going to make it anything but a 90. With exhaust boring you might make an inline 80.

[rideordie]

my 90 has 3 carbs and 6 sparkplugs straight up and down?

[Droll]

Technically speaking the intake which is the entire front half of the block would fit with enough work

If he got the dist type block, the front itself are an easy bolt on but ...

The rest of the conversion are NOT an easy bolt on, no doubt it could be done but only by someone who got the knowhow..

Bolt ons for a inline 90 will prove to be a lesson in frustration. If you can not learn to love it like it is set it free and find abetter match for your use.

The 90 L6 can be made into something a bit more exiting ( have done it ) but it does require some major work...
Disassemble the whole engine.

• Enlarge intake and exhaust ports + add some power ports ( I did not add any PP )
• Change reeds to the 1500/1400 banjo type .
• Find some god 1500/1400 carbs
• Get a set of new power port pistons ( IF you did cut the PP groves in the cylinder )

Best advice would be to find a decent 1500/1400 or a late model 115 ( 1984 > ) and just replace the whole engine , THATS an easy bolt on ..


Just for your information...

THIS is a 1500 Twister aka Mercury T2 .... or will be the when it's restored....


Arne Kjetil

[daveswaves]

Arne is right, things can be done, I just dont want you to think you can just hog out the exhaust and make magic.

[rideordie]

yea i no boring exhaust want do nothing but sound bettter im hopin my boat will run atleast 60. ima keep the motor for a while till i find a bigger one. but borin the exhaust wont slow it down or hurt anything should it?

[daveswaves]

Honestly, boring your exhaust will not do anything except make it run worse. The inline 90 is restricted on the intake side, not the exhaust side. Power ported pistons, finger ports in the block and 10 10 reed blocks will all make a difference. Like arne said though, you will be further ahead by obtaining a 140 or 150 block and bolting it on. We do modifications because we are prepared to live with the results, sometimes not pretty. I have spare parts and if I pooch a piston I just change it, no big deal.
60 is tough to get with a 90 hp inline, the boat setup and the prop must be exactly right. I don,t want to discourage you from experimenting, just not arbitrarily. And boring the exhaust is arbitrary.

[rideordie]

Wat size prop would u perfer to run on it? How far should i set the prop shaft under the boat like 3-4 inches?

[daveswaves]

Wat size prop would u perfer to run on it? How far should i set the prop shaft under the boat like 3-4 inches?

Need some more info on your boat, length, type of hull, current set up on the boat, what rpm are you turning at WOT, are you able to lift the bow out of the water when you trim up, what is your hole shot like. A picture of the stern of your boat taken from behind the prop would tell us whether you need to move up or down on the transom. The 18P is certainly within the range of the 90 but if you are not getting the rpm you are not going to get the best out of it. Also the make of the prop or at least a good pic of it.

[delawarerick]

When Dave S built me a L6 I started with a 90 mariner with a handfull of hrs(low) that said the first thing he did was get rid of the 90 block and got a late model 115. What he did from there I know not but this engine is strong. But point of response was he did not use the 90 block. Rick

[daveswaves]

When Dave S built me a L6 I started with a 90 mariner with a handfull of hrs(low) that said the first thing he did was get rid of the 90 block and got a late model 115. What he did from there I know not but this engine is strong. But point of response was he did not use the 90 block. Rick

mode

EXACTLY Rick. The late model 115 is power ported and one snappy block.

[rideordie]

ok ima post more pics this week on it. i had alot of ppl tell me that my 90 inline would out run the 140 johnson i had wats the belief on that?

[Mark75H]

ok ima post more pics this week on it. i had alot of ppl tell me that my 90 inline would out run the 140 johnson i had wats the belief on that?

Little or none